WESTFIELD CAPITAL MANAGEMENT CO LP — 13F Holdings & Portfolio
CIK 1177719 · latest 13F-HR filed 2026-08-12
WESTFIELD CAPITAL MANAGEMENT CO LP manages $30.61B in 13F-reported U.S. long-equity assets across 278 reported positions as of its 2026-06-30 13F-HR filing. Its largest disclosed positions are NVDA (5.28%), GOOGL (4.38%), ASND (4.18%). Quarter over quarter (versus its 2026-03-31 filing) the fund opened 69 new positions and exited 41; 128 existing positions rose in reported value and 60 fell.
AUM is the long-equity portion reported on Form 13F-HR. Excludes short positions, cash, foreign holdings, and non-13F assets. For informational purposes only — not investment advice.
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AUM
$30.61B
Long-equity book
Holdings
278
Distinct positions
Period
2026-06-30
Filed 2026-08-12
Q/Q Activity
+69 / −41 / ↑128 / ↓60
vs 2026-03-31
Portfolio charts
Latest quarter's book, what changed, and how the portfolio has evolved across filings.
Portfolio map
Q2 26NVDA5.3% · +28%GOOGL4.4% · +85%ASND4.2% · -0%AXON2.7% · +40%FTAI2.6% · +3%AAPL2.4% · -18%FIX2.3% · -43%AVGO1.9% · +26%MSFT1.7% · -14%HWM1.7% · -16%LLY1.6% · +14%SNOW1.4% · +142%MKSI1.3% · -11%META1.2% · +29%VRT1.2% · -45%GEV0.9% · +21%MU0.9% · +127%V0.9% · +67%APG0.9% · +33%TPR0.8% · -23%RYTM0.8% · +14%LTH0.8% · +88%TSM0.8% · +4%GH0.8% · +19%PWR0.8% · -0%NOW0.8% · +70%AMD0.8% · +7930%PANW0.7% · +3%WWD0.7% · +93%ANET0.7% · +69%LPLA0.7% · -10%RKLB0.7% · -33%AS0.7% · +61%NET0.7% · +54%AMZN0.7% · -62%GRMN0.7% · +129%RRX0.7% · +26%NTRA0.6% · -4%MS0.6% · +89%CRS0.6% · newIOT0.6% · +15%DT0.6% · +1%CAT0.6% · +440%SMTC0.6% · -12%AME0.6% · -3%ABVX0.6% · -1%INSM0.6% · +11%ITT0.5% · +18%GILD0.5% · +4724%PTCT0.5% · newBROS0.5% · -6%CELH0.5% · +41%MTZ0.5% · +50%SN0.5% · +12%CSX0.5% · +90%FCFS0.5% · +60%KEY0.5% · +25%MCHP0.5% · +69%AMAT0.5% · +21865%MAR0.5% · +160%
Box size = position value; colour = change in share count vs the prior quarter (green added, red trimmed, blue new). Rows split across managers or share classes are combined. Click a box for the stock.
Sector mix
% of 13F valueSectors from each company's SIC code; funds and ETFs grouped separately.
Biggest buys and sells
Q1 26 → Q2 26Estimated from the change in shares held × the quarter-end price, so price moves alone don't count as buying. New and exited positions are included.
13F assets and positions
4 quartersTotal reported 13F value (bars) and number of positions (line) each quarter. Value changes include market moves.
Top positions over time
reported valueThe eight largest current positions, valued at each quarter end.
Concentration
top 10 = 29.2%Cumulative share of the portfolio held in the largest N positions.
Firm Profile & Key People
As of the firm’s regulatory filing dated Jul 1, 2026.
Assets Under Management
$29.81B
Discretionary
$29.81B
Employees
63
Accounts
9,391
Address
ONE FINANCIAL CENTER
BOSTON, MA 02111
BOSTON, MA 02111
Phone
(617) 428-7100
| Name | Title / Status | Ownership | Type |
|---|---|---|---|
| KEARNEY, KATHRYN, ANN Control | PARTNER, CHIEF FINANCIAL OFFICER AND CHIEF COMPLIANCE OFFICER since Dec 2012 | Less than 5% | Individual |
| LEE, RICHARD, DAILYONG Control | CHIEF INVESTMENT OFFICER since Jan 2016 | Less than 5% | Individual |
| MONTGOMERY, JOHN, MORGEN Control | CHIEF OPERATING OFFICER since Jan 2015 | Less than 5% | Individual |
| MUGGIA, WILLIAM, ALBERT Control | CHIEF EXECUTIVE OFFICER AND CHIEF INVESTMENT OFFICER since Nov 2001 | Less than 5% | Individual |
| WMS GENERAL PARTNER LLC Control | GENERAL PARTNER since Jun 2008 | Less than 5% | Domestic Entity |
| WMS MANAGEMENT LLC Control | LIMITED PARTNER since Jun 2008 | 75% or more | Domestic Entity |
| MG AM HOLDING, INC. | LIMITED PARTNER since Apr 2025 | 10–25% | Domestic Entity |
| FLORES, ROBERT, TIZOC Control Indirect owner via WMS MANAGEMENT LLC | MEMBER OF MANAGEMENT COMMITTEE since Jan 2013 | General partner, trustee or manager | Individual |
| KEARNEY, KATHRYN, ANN Control Indirect owner via WMS MANAGEMENT LLC | MEMBER OF MANAGEMENT COMMITTEE since Jan 2021 | General partner, trustee or manager | Individual |
| LEE, RICHARD, DAILYONG Control Indirect owner via WMS MANAGEMENT LLC | MEMBER OF MANAGEMENT COMMITTEE since Jan 2013 | General partner, trustee or manager | Individual |
| MONTGOMERY, JOHN, MORGEN Control Indirect owner via WMS MANAGEMENT LLC | MEMBER OF MANAGEMENT COMMITTEE since Jan 2013 | General partner, trustee or manager | Individual |
| MOSCARDELLI, JUSTIN, MATTHEW Control Indirect owner via WMS MANAGEMENT LLC | MEMBER OF MANAGEMENT COMMITTEE since Jan 2024 | General partner, trustee or manager | Individual |
| MUGGIA, WILLIAM, ALBERT Control Indirect owner via WMS MANAGEMENT LLC | MEMBER OF MANAGEMENT COMMITTEE since Jun 2008 | 50–75% | Individual |
| MULLER, JENNY, ANN Control Indirect owner via WMS MANAGEMENT LLC | MEMBER OF MANAGEMENT COMMITTEE since Jan 2021 | General partner, trustee or manager | Individual |
| RENNA, MATTHEW, ROBERT Control Indirect owner via WMS MANAGEMENT LLC | MEMBER OF MANAGEMENT COMMITTEE since Jan 2021 | General partner, trustee or manager | Individual |
| WMS MANAGEMENT LLC Control Indirect owner via WMS GENERAL PARTNER LLC | SOLE MEMBER since Jun 2008 | 75% or more | Domestic Entity |
| MONEX GROUP, INC. Indirect owner via MG AM HOLDING, INC. | SHAREHOLDER OF MG AM HOLDING, INC. since Apr 2025 | General partner, trustee or manager | Foreign Entity |
Ownership & leadership changes · 16 filings with changes since Dec 21, 2011
From the firm’s regulatory filings, newest first. Dates are filing dates: a filing that no longer lists someone does not say when they left.
Oct 10, 2025 filing
- AddedMG AM HOLDING, INC. · LIMITED PARTNER · 10–25%
- AddedMONEX GROUP, INC. · SHAREHOLDER OF MG AM HOLDING, INC. · General partner, trustee or manager · indirect via MG AM HOLDING, INC.
Apr 30, 2025 filing
- No longer listedMEYERS, ETHAN, JOSEPH · MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC · control person · listed since Jan 2013
Mar 7, 2024 filing
- AddedMOSCARDELLI, JUSTIN, MATTHEW · MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC · control person
- ChangedLEE, RICHARD, DAILYONG · title CO-CHIEF INVESTMENT OFFICER → CHIEF INVESTMENT OFFICER
Mar 10, 2023 filing
- ChangedLEE, RICHARD, DAILYONG · title DEPUTY CHIEF INVESTMENT OFFICER → CO-CHIEF INVESTMENT OFFICER
Mar 8, 2021 filing
- AddedKEARNEY, KATHRYN, ANN · MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC · control person
- AddedMULLER, JENNY, ANN · MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC · control person
- AddedRENNA, MATTHEW, ROBERT · MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC · control person
May 8, 2020 filing
- No longer listedTHOMPSON, DAVID, HAMLEN · MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC · control person · listed since Jan 2011
Jul 9, 2019 filing
- No longer listedJACOBS, BRUCE, NATHAN · MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC · control person · listed since Jan 2013
Mar 12, 2018 filing
- ChangedKEARNEY, KATHRYN, ANN · title CHIEF FINANCIAL OFFICER AND CHIEF COMPLIANCE OFFICER → PARTNER, CHIEF FINANCIAL OFFICER AND CHIEF COMPLIANCE OFFICER
Mar 14, 2016 filing
- AddedLEE, RICHARD, DAILYONG · DEPUTY CHIEF INVESTMENT OFFICER · Less than 5% · control person
- ChangedMUGGIA, WILLIAM, ALBERT · title CEO → CHIEF EXECUTIVE OFFICER AND CHIEF INVESTMENT OFFICER
Dec 4, 2015 filing
- No longer listedMCAULEY, HELEN, LEE · CHIEF COMPLIANCE OFFICER · Less than 5% · control person · listed since Jan 2013
- ChangedKEARNEY, KATHRYN, ANN · title CHIEF FINANCIAL OFFICER → CHIEF FINANCIAL OFFICER AND CHIEF COMPLIANCE OFFICER
Older changes (7 more filings)
Mar 18, 2015 filing
- AddedMONTGOMERY, JOHN, MORGEN · CHIEF OPERATING OFFICER · Less than 5% · control person
Oct 15, 2014 filing
- ChangedKEARNEY, KATHRYN, ANN · now a control person
- ChangedMCAULEY, HELEN, LEE · now a control person
Aug 19, 2014 filing
- No longer listedFEAREY, MORTON · MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC · control person · listed since Jun 2008
Aug 28, 2013 filing
- ChangedFEAREY, MORTON · indirect via WMS MANAGEMENT LLC: now a control person
- ChangedFLORES, ROBERT, TIZOC · indirect via WMS MANAGEMENT LLC: now a control person
- ChangedJACOBS, BRUCE, NATHAN · indirect via WMS MANAGEMENT LLC: now a control person
- ChangedLEE, RICHARD, DAILYONG · indirect via WMS MANAGEMENT LLC: now a control person
- ChangedMEYERS, ETHAN, JOSEPH · indirect via WMS MANAGEMENT LLC: now a control person
- ChangedMONTGOMERY, JOHN, MORGEN · indirect via WMS MANAGEMENT LLC: now a control person
- ChangedTHOMPSON, DAVID, HAMLEN · indirect via WMS MANAGEMENT LLC: now a control person
Mar 14, 2013 filing
- AddedFLORES, ROBERT, TIZOC · MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC
- AddedJACOBS, BRUCE, NATHAN · MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC
- AddedLEE, RICHARD, DAILYONG · MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC
- AddedMEYERS, ETHAN, JOSEPH · MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC
- AddedMONTGOMERY, JOHN, MORGEN · MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC
- ChangedFEAREY, MORTON · indirect via WMS MANAGEMENT LLC: title CLASS A & B MEMBER, MEMBER OF MANAGEMENT COMMITTEE → MEMBER OF MANAGEMENT COMMITTEE
- ChangedMUGGIA, WILLIAM, ALBERT · indirect via WMS MANAGEMENT LLC: title CLASS A MEMBER, MEMBER OF MANAGEMENT COMMITTEE → MEMBER OF MANAGEMENT COMMITTEE
- ChangedTHOMPSON, DAVID, HAMLEN · indirect via WMS MANAGEMENT LLC: title CLASS B MEMBER, MEMBER OF MANAGEMENT COMMITTEE → MEMBER OF MANAGEMENT COMMITTEE
Jan 11, 2013 filing
- No longer listedDIGRAVIO, KAREN, ANN · CFO, CHIEF COMPLIANCE OFFICER · Less than 5% · control person · listed since Jan 2001
- No longer listedDIGRAVIO, KAREN, ANN · CLASS B MEMBER, MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC · listed since Jun 2008
- AddedKEARNEY, KATHRYN, ANN · CHIEF FINANCIAL OFFICER · Less than 5%
- AddedMCAULEY, HELEN, LEE · CHIEF COMPLIANCE OFFICER · Less than 5%
Dec 21, 2011 filing (earliest on record)
- ListedDIGRAVIO, KAREN, ANN · CFO, CHIEF COMPLIANCE OFFICER · Less than 5% · control person
- ListedDIGRAVIO, KAREN, ANN · CLASS B MEMBER, MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC
- ListedFEAREY, MORTON · CLASS A & B MEMBER, MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC
- ListedMUGGIA, WILLIAM, ALBERT · CEO · Less than 5% · control person
- ListedMUGGIA, WILLIAM, ALBERT · CLASS A MEMBER, MEMBER OF MANAGEMENT COMMITTEE · 50–75% · indirect via WMS MANAGEMENT LLC · control person
- ListedTHOMPSON, DAVID, HAMLEN · CLASS B MEMBER, MEMBER OF MANAGEMENT COMMITTEE · General partner, trustee or manager · indirect via WMS MANAGEMENT LLC
- ListedWMS GENERAL PARTNER LLC · GENERAL PARTNER · Less than 5% · control person
- ListedWMS MANAGEMENT LLC · LIMITED PARTNER · 75% or more · control person
- ListedWMS MANAGEMENT LLC · SOLE MEMBER · 75% or more · indirect via WMS GENERAL PARTNER LLC · control person
Quarter-over-quarter changes
Changes in reported position value vs the prior 13F, so price moves count too, not just shares bought or sold.
Top Adds (vs prior quarter)
- ALPHABET INC$757.0M +129.9%
- NVIDIA CORPORATION$515.0M +46.7%
- AXON ENTERPRISE INC$384.2M +85.0%
- SNOWFLAKE INC$333.5M +307.7%
- ADVANCED MICRO DEVICES INC$233.1M +22828.8%
Top Trims
- AMAZON COM INC-$269.9M -56.8%
- NETFLIX INC.-$168.6M -89.0%
- MARVELL TECHNOLOGY INC-$161.8M -97.6%
- BLOOM ENERGY CORP-$158.2M -98.0%
- COMFORT SYS USA INC-$151.8M -17.7%
New Positions
- ASCENDIS PHARMA A/S$1.28B
- CARPENTER TECHNOLOGY CORP$193.7M
- PTC THERAPEUTICS INC$159.6M
- LAM RESEARCH CORP$123.8M
- ASML HLDG NV$114.3M
Exited Positions
- ASCENDIS PHARMA A/S$1.10B
- KRATOS DEFENSE & SEC SOLUTIO$110.2M
- AMERICAN INTL GROUP INC$109.8M
- PALANTIR TECHNOLOGIES INC$98.7M
- GENEDX HOLDINGS CORP$87.9M
WESTFIELD CAPITAL MANAGEMENT CO LP Holdings (2026-06-30)
Rows tagged Call or Put are options positions, not stock. Per SEC Form 13F-HR, Value shows the notional value of the underlying (shares covered × underlying price), not the option premium. Shares is the number of underlying shares covered (divide by 100 for contract count).
| # | Issuer | Symbol | CUSIP | Value | Weight | Shares | Type |
|---|---|---|---|---|---|---|---|
| 1 | NVIDIA CORPORATION | NVDA | 67066G104 | $1.62B | 5.28% | 8,079,926 | SH |
| 2 | ALPHABET INC | GOOGL | 02079K305 | $1.34B | 4.38% | 3,749,074 | SH |
| 3 | ASCENDIS PHARMA A/S | ASND | K08588103 | $1.28B | 4.18% | 4,798,267 | SH |
| 4 | AXON ENTERPRISE INC | AXON | 05464C101 | $836.1M | 2.73% | 1,491,490 | SH |
| 5 | FTAI AVIATION LTD | FTAI | G3730V105 | $794.6M | 2.60% | 2,937,228 | SH |
| 6 | APPLE INC | AAPL | 037833100 | $744.7M | 2.43% | 2,573,774 | SH |
| 7 | COMFORT SYS USA INC | FIX | 199908104 | $704.5M | 2.30% | 355,433 | SH |
| 8 | BROADCOM INC | AVGO | 11135F101 | $588.0M | 1.92% | 1,556,522 | SH |
| 9 | MICROSOFT CORP | MSFT | 594918104 | $527.6M | 1.72% | 1,414,294 | SH |
| 10 | HOWMET AEROSPACE INC | HWM | 443201108 | $516.6M | 1.69% | 1,921,476 | SH |
| 11 | ELI LILLY & CO | LLY | 532457108 | $499.8M | 1.63% | 416,732 | SH |
| 12 | SNOWFLAKE INC | SNOW | 833445109 | $441.9M | 1.44% | 1,736,498 | SH |
| 13 | MKS INC. | MKSI | 55306N104 | $413.0M | 1.35% | 928,617 | SH |
| 14 | META PLATFORMS INC | META | 30303M102 | $381.6M | 1.25% | 677,389 | SH |
| 15 | VERTIV HOLDINGS CO | VRT | 92537N108 | $353.0M | 1.15% | 1,054,361 | SH |
| 16 | GE VERNOVA INC | GEV | 36828A101 | $283.5M | 0.93% | 241,272 | SH |
| 17 | MICRON TECHNOLOGY INC | MU | 595112103 | $263.4M | 0.86% | 228,203 | SH |
| 18 | VISA INC | V | 92826C839 | $261.6M | 0.85% | 762,403 | SH |
| 19 | API GROUP CORP | APG | 00187Y100 | $260.6M | 0.85% | 6,154,301 | SH |
| 20 | TAPESTRY INC | TPR | 876030107 | $248.3M | 0.81% | 1,696,569 | SH |
| 21 | RHYTHM PHARMACEUTICALS INC | RYTM | 76243J105 | $245.2M | 0.80% | 2,208,595 | SH |
| 22 | LIFE TIME GROUP HOLDINGS INC | LTH | 53190C102 | $244.8M | 0.80% | 5,995,254 | SH |
| 23 | TAIWAN SEMICONDUCTOR MANUFAC | TSM | 874039100 | $244.0M | 0.80% | 510,999 | SH |
| 24 | GUARDANT HEALTH INC | GH | 40131M109 | $243.7M | 0.80% | 1,624,459 | SH |
| 25 | QUANTA SVCS INC | PWR | 74762E102 | $238.7M | 0.78% | 331,489 | SH |
| 26 | SERVICENOW INC | NOW | 81762P102 | $235.7M | 0.77% | 2,373,711 | SH |
| 27 | ADVANCED MICRO DEVICES INC | AMD | 007903107 | $234.2M | 0.76% | 403,081 | SH |
| 28 | PALO ALTO NETWORKS INC | PANW | 697435105 | $224.5M | 0.73% | 658,331 | SH |
| 29 | WOODWARD INC | WWD | 980745103 | $224.0M | 0.73% | 526,505 | SH |
| 30 | ARISTA NETWORKS INC | ANET | 040413205 | $223.7M | 0.73% | 1,317,096 | SH |
| 31 | LPL FINL HLDGS INC | LPLA | 50212V100 | $223.4M | 0.73% | 793,061 | SH |
| 32 | ROCKET LAB CORP | RKLB | 773121108 | $214.9M | 0.70% | 2,114,005 | SH |
| 33 | AMER SPORTS INC | AS | G0260P102 | $214.7M | 0.70% | 6,344,923 | SH |
| 34 | CLOUDFLARE INC | NET | 18915M107 | $204.9M | 0.67% | 835,502 | SH |
| 35 | AMAZON COM INC | AMZN | 023135106 | $204.9M | 0.67% | 859,806 | SH |
| 36 | GARMIN LTD | GRMN | H2906T109 | $204.5M | 0.67% | 861,057 | SH |
| 37 | REGAL REXNORD CORPORATION | RRX | 758750103 | $204.1M | 0.67% | 856,976 | SH |
| 38 | NATERA INC | NTRA | 632307104 | $197.2M | 0.64% | 726,628 | SH |
| 39 | MORGAN STANLEY | MS | 617446448 | $196.6M | 0.64% | 940,560 | SH |
| 40 | CARPENTER TECHNOLOGY CORP | CRS | 144285103 | $193.7M | 0.63% | 314,082 | SH |
| 41 | SAMSARA INC | IOT | 79589L106 | $189.3M | 0.62% | 5,836,459 | SH |
| 42 | DYNATRACE INC | DT | 268150109 | $186.6M | 0.61% | 4,249,229 | SH |
| 43 | CATERPILLAR INC | CAT | 149123101 | $186.0M | 0.61% | 174,620 | SH |
| 44 | SEMTECH CORP | SMTC | 816850101 | $183.4M | 0.60% | 1,133,268 | SH |
| 45 | AMETEK INC | AME | 031100100 | $175.1M | 0.57% | 723,798 | SH |
| 46 | ABIVAX SA | ABVX | 00370M103 | $174.5M | 0.57% | 1,309,825 | SH |
| 47 | INSMED INC | INSM | 457669307 | $173.7M | 0.57% | 1,629,561 | SH |
| 48 | ITT INC | ITT | 45073V108 | $164.2M | 0.54% | 830,382 | SH |
| 49 | GILEAD SCIENCES INC | GILD | 375558103 | $162.7M | 0.53% | 1,287,629 | SH |
| 50 | PTC THERAPEUTICS INC | PTCT | 69366J200 | $159.6M | 0.52% | 1,956,156 | SH |
Filing History
| Period | Filed | AUM | Holdings | Accession |
|---|---|---|---|---|
| 2026-06-30 | 2026-08-12 | $30.61B | 278 | 0001177719-26-000021 |
| 2026-03-31 | 2026-05-11 | $23.83B | 250 | 0001177719-26-000018 |
| 2025-12-31 | 2026-02-11 | $24.01B | 241 | 0001177719-26-000005 |
| 2025-09-30 | 2025-11-12 | $23.83B | 235 | 0001177719-25-000003 |
Filer Information
Address
ONE FINANCIAL CENTER23RD FLOOR
BOSTON, MA 02111
Phone
(617) 428-7100
Filing Manager
WESTFIELD CAPITAL MANAGEMENT CO LP
BOSTON, MA
Signatory
Brandi McMahon
Vice President, Deputy Chief Compliance Officer