WESTFIELD CAPITAL MANAGEMENT CO LP

New 13F: Q3 2026

WESTFIELD CAPITAL MANAGEMENT CO LP filed its 13F for the quarter ended Sep 30, 2026 on Nov 12, 2026: $13.2B in AI and semiconductor stocks; $1.9B bought and $1.6B sold against its previous quarter, at the quarter's prices.

• trimmed Comfort Systems USA (FIX): -$487.2M at the quarter's price
• sold out of Vertiv (VRT): -$353.0M at the quarter's price
• opened a position in Palantir (PLTR): +$246.9M at the quarter's price
• added to Micron (MU): +$167.9M at the quarter's price
• opened a position in Celestica (CLS): +$152.1M at the quarter's price
• added to Fabrinet (FN): +$120.3M at the quarter's price
• added to MongoDB (MDB): +$119.4M at the quarter's price
• added to Meta Platforms (META): +$112.1M at the quarter's price
• added to TTM Technologies (TTMI): +$87.3M at the quarter's price
• trimmed Apple (AAPL): -$76.6M at the quarter's price

WESTFIELD CAPITAL MANAGEMENT CO LP declares $12.9B in 74 of the 230 AI and semiconductor companies tracked on Silicon Floor, as of Jun 30, 2026 (Form 13F): 42.2% of its $30.6B 13F portfolio. Largest positions: NVIDIA (0.03% of the company) and Alphabet (0.03% of the company). Over time: $12.9B in 74 of these companies for Q2 2026, against $3.3B in 29 for Q4 2019 (13F values as filed). From Mar 31, 2026 to Jun 30, 2026, it bought $3.8B and sold $3.4B of these stocks, at the quarter's prices: a net purchase of $387.0M; 19 new positions and 3 sold out.

Performance of its AI picks

The AI-stock portfolio WESTFIELD CAPITAL MANAGEMENT CO LP declared for Q2 2026 (Form 13F), held unchanged, returned -0.9% in Q3 2026 (SMH -7.1%), ranked 1371 of 1,785 active managers. Over the last 12 months, quarter after quarter: +43.2% (SMH +87.2%), ranked 229 of 1,360 active managers. Over three years, quarter after quarter: +226.1% (SMH +325.8%), ranked 69 of 784 active managers. Over five years, quarter after quarter: +232.2% (SMH +389.8%), ranked 53 of 535 active managers. Trades made during a quarter are not visible in 13F filings: this measures its declared picks, not the return of its funds.

Largest positions

1. NVIDIA (NVDA): $1.6B, 8.1M shares, 0.03% of the company, 5.3% of its portfolio (added this quarter); held since Q4 2019 or earlier
2. Alphabet (GOOGL): $1.4B, 3.8M shares, 0.03% of the company, 4.4% of its portfolio (added this quarter); held since Q4 2019 or earlier
3. Apple (AAPL): $744.7M, 2.6M shares, 0.02% of the company, 2.4% of its portfolio (trimmed this quarter); held since Q4 2019 or earlier
4. Comfort Systems USA (FIX): $704.5M, 355,433 shares, 1.0% of the company, 2.3% of its portfolio (trimmed this quarter); held since Q4 2023
5. Broadcom (AVGO): $588.0M, 1.6M shares, 0.03% of the company, 1.9% of its portfolio (added this quarter); held since Q4 2019 or earlier
6. Microsoft (MSFT): $527.6M, 1.4M shares, 0.02% of the company, 1.7% of its portfolio (trimmed this quarter); held since Q4 2019 or earlier
7. Snowflake (SNOW): $441.9M, 1.7M shares, 0.49% of the company, 1.4% of its portfolio (added this quarter); held since Q2 2025
8. MKS (MKSI): $413.0M, 928,617 shares, 1.4% of the company, 1.3% of its portfolio (trimmed this quarter); held since Q3 2025
9. Meta Platforms (META): $381.6M, 677,389 shares, 0.03% of the company, 1.2% of its portfolio (added this quarter); held since Q1 2023
10. Vertiv (VRT): $353.0M, 1.1M shares, 0.27% of the company, 1.2% of its portfolio (trimmed this quarter); held since Q3 2023
11. GE Vernova (GEV): $283.5M, 241,272 shares, 0.09% of the company, 0.93% of its portfolio (added this quarter); held since Q3 2024
12. Micron (MU): $263.4M, 228,203 shares, 0.02% of the company, 0.86% of its portfolio (added this quarter); held since Q1 2026
13. TSMC (TSM): $244.0M, 510,999 shares, 0.01% of the company, 0.80% of its portfolio (added this quarter); held since Q1 2024
14. Quanta Services (PWR): $238.7M, 331,489 shares, 0.22% of the company, 0.78% of its portfolio (trimmed this quarter); held since Q2 2025
15. ServiceNow (NOW): $235.7M, 2.4M shares, 0.23% of the company, 0.77% of its portfolio (added this quarter); held since Q4 2019 or earlier

By sector

• Cloud & AI giants: $3.2B, 25% of its AI holdings, 5 companies
• Chips & compute: $3.1B, 24% of its AI holdings, 12 companies
• Power, electrical & cooling: $2.2B, 17% of its AI holdings, 15 companies
• AI software: $1.4B, 11% of its AI holdings, 8 companies
• Equipment & test: $1.1B, 8% of its AI holdings, 8 companies
• Interconnect: $754.2M, 6% of its AI holdings, 7 companies
• Memory & storage: $459.3M, 4% of its AI holdings, 4 companies
• Optics: $310.1M, 2% of its AI holdings, 6 companies
• Servers & systems: $277.4M, 2% of its AI holdings, 4 companies
• Neoclouds & data centers: $166.2M, 1% of its AI holdings, 5 companies

Biggest changes this quarter

• added to Alphabet (GOOGL): +$618.0M at the quarter's price
• trimmed Comfort Systems USA (FIX): -$526.2M at the quarter's price
• trimmed Marvell (MRVL): -$494.5M at the quarter's price
• trimmed Bloom Energy (BE): -$357.5M at the quarter's price
• added to NVIDIA (NVDA): +$352.7M at the quarter's price
• trimmed Amazon (AMZN): -$338.4M at the quarter's price
• trimmed Astera Labs (ALAB): -$295.0M at the quarter's price
• trimmed Vertiv (VRT): -$290.4M at the quarter's price
• added to Snowflake (SNOW): +$259.0M at the quarter's price
• added to AMD (AMD): +$231.2M at the quarter's price

Other holders of this size

• a16z Capital Management, L.L.C.: $12.9B in 2 of these companies
• CIBC Asset Management Inc: $13.1B in 173 of these companies
• State of Tennessee, Department of Treasury: $12.7B in 159 of these companies
• COMMONWEALTH EQUITY SERVICES, LLC: $13.3B in 197 of these companies
• SOFTBANK GROUP CORP.: $12.7B in 3 of these companies
• Retirement Systems of Alabama: $13.3B in 126 of these companies
• SEB Asset Management AB: $12.7B in 104 of these companies
• Connor, Clark & Lunn Investment Management Ltd.: $13.4B in 138 of these companies

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