Mackenzie Tatananni and Nate Wolf

What do an energy supplier and the nation's largest healthcare company have in common? Nothing apparently, seeing as one name topped the list of the S&P 500's best performers this month and the other ended up at the bottom. Here are the Index's top May winners and losers as of Thursday's close:

NRG Energy led the Index, with shares rising 41% this month. Most of those gains came just after the energy supplier's May 12 announcement that it had acquired 18 natural gas-generation plants, doubling the company's power-generation capacity. NRG, which also handily beat first-quarter earnings and revenue expectations, is beefing up its fleet of facilities to meet the demand of energy-hungry data centers. "This acquisition transforms NRG's generation fleet," said CEO Larry Coben.

Sticking with the energy companies powering U.S. data centers, Constellation Energy was another winner in May, gaining 35.8%. The clean-energy company's first-quarter earnings missed expectations, but an upbeat full-year forecast buoyed the stock. Management expressed confidence in Constellation's nuclear fleet and continued demand from the technology sector. The stock also got a bump after President Donald Trump signed executive orders on May 23 accelerating the construction of nuclear reactors.

Seagate Technology Holdings rose 29.8%. The data-storage company came into May with momentum -- it was the S&P 500's top performer on April 30 -- and shares continued to climb. At a May 22 investor event, Seagate announced that its board had boosted the company's share-repurchase authorization to $5 billion.

Super Micro Computer climbed 29.2% -- bad news for the investors who made it the most-shorted stock in the S&P 500 in April. The server maker got a boost on May 14 when it announced both a $20 billion deal with Saudi Arabian data center firm DataVolt, and the shipment of high-density servers powered by Advanced Micro Devices processors. Super Micro Computer's stock has been notoriously volatile since the start of last year.

Insulet jumped 28.4% in May. The stock spiked on May 9, when the insulin pump maker breezed past Wall Street's first-quarter earnings expectations, and raised guidance for the rest of 2025. The stock has been on a tear since last August, when the Food and Drug Administration approved its Insulet's Omnipod 5 Insulin System for adults with Type 2 diabetes.

The worst performer by a wide margin was UnitedHealth Group, which declined 27.5% this month. Its troubles began in earnest at the end of April, when the healthcare firm slashed full-year guidance, triggering a selloff. A bad situation got even worse on May 13 when the company pulled its outlook altogether, citing higher-than-expected medical expenditures. In a brutal one-two punch, the company then announced the resignation of former CEO Andrew Witty, citing "personal reasons."

Eli Lilly declined 19.6%. The stock began to slide on May 1 after Novo Nordisk unveiled a significant deal with CVS Health's pharmacy-benefit manager that granted patients easier access to Wegovy, putting pressure on Lilly and its competing GLP-1 inhibitor Zepbound. Remarks from President Trump about pharmaceutical tariffs and the resulting uncertainty have also caused shares to waver.

Becton Dickinson slumped 16.1%. The company reported second-quarter earnings on May 1 and cautioned investors that tariffs would likely hurt fiscal-year profits. Management noted that global funding cuts had taken a hit on research instrument sales. The flood of bad news caused the stock to see a historic decline. Shares fell 18.1% on May 1 alone, marking the worst day since June 1999, according to Dow Jones Market Data.

Copart fell 15.7%. Shares of the online auto auctioneer tumbled this month after sales missed analysts' expectations, although earnings were in line with forecasts. The resulting selloff indicated that Copart's latest financials raised a significant red flag for investors, who had lofty expectations heading into the report and simply expected more.

Fair Isaac saw a 15.3% drop in May. The stock has been beaten down in recent weeks amid comments from Federal Housing Finance Agency Director Bill Pulte. While there is little Pulte can do to address most of his gripes about FICO's pricing strategy, the director has shown interest in shifting from tri-merge credit reports, which use FICO Scores from all three major credit bureaus, to bi-merge reports that only use two -- potentially a way to reduce costs by spurring on competition among credit-rating companies.

Write to Mackenzie Tatananni at mackenzie.tatananni@barrons.com and Nate Wolf at nate.wolf@barrons.com

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May 30, 2025 15:29 ET (19:29 GMT)

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