This Was the Top-Performing Stock in the S&P 500 in August 2025

The S&P 500 (^GSPC 0.01%) market index gained 1.9% in August 2025. That may not sound like much, but it's actually a pretty robust gain. Keep that up for 12 straight months and you'd have a total return of 25.3%. Not every component of the index carried its weight last month. For example, Super Micro Computer (SMCI 3.30%) shares dropped 29.6% lower and Gartner (IT 1.15%) took a 25.8% haircut. But I'm not here to dwell on the S&P 500's weakest performers. Instead, let's celebrate UnitedHealth Group (UNH 0.94%), which delivered the index's strongest August showing with a 24.2% price gain. UnitedHealth nursed some serious wounds when July ended It should be noted that UnitedHealth entered August on a low note. The health insurance colossus posted a 20% price drop in July because of a disappointing bottom-line result and modest guidance in the second quarter of 2025. The report was published on July 29, giving bearish investors just enough time to react with a brutal stock price cut before the end of that month. Taken together, the plunge-and-jump combination resulted in a 0.7% price drop over two months. That said, UnitedHealth's recovery was based on two separate events on the same day. Image source: Getty Images. Two big announcements powered the August surge On Aug. 15, Warren Buffett's Berkshire Hathaway (BRK.A -0.39%) (BRK.B -0.45%) filed its latest trade reports, including a new position of 5 million UnitedHealth shares. Berkshire isn't known for explaining its stock picks in great detail, and Buffett himself probably didn't hit the proverbial buy button in this case. Warren Buffett's retirement is coming up soon, and his trusted lieutenants should carry a heavier load now. Still, investors saw the trade as a top-of-the-line approval of UnitedHealth's investment value. Berkshire buys tend to have that effect, with or without Buffett's direct involvement. And on the same day, UnitedHealth closed the $3.3 billion acquisition of home health expert Amedisys. The Berkshire position probably made a bigger difference, because the Amedisys deal had been pending since the summer of 2023.