S&P 500: The Winners and Losers of October 2025
In October the S&P 500 continued its hot streak, rising about 2.3% on the month – marking its sixth straight monthly gain. This extended a rally that began in spring (the index is now roughly +16% YTD) as strong corporate earnings and enthusiasm for AI-related technologies drove broad gains.
This steady rise follows a year marked by sharp swings. After a 2.7% gain in January, the benchmark struggled through February and March before regaining traction in April. Since then, it has notched consistent monthly gains, underscoring renewed investor confidence and persistent momentum across sectors.
In this article we review October’s biggest winners and laggards in the S&P 500 and explain the company-specific news behind their moves.
Top S&P 500 Performers in October
Among the top performers were Advanced Micro Devices (AMD), Micron Technology (MU), and Teradyne (TER). Each saw double-digit gains as industry-specific catalysts and positive earnings drove their rally.
Advanced Micro Devices ($AMD) +58.30%
AMD was October’s standout performer, soaring nearly 60% after announcing a landmark multi-year partnership with OpenAI that positions the chipmaker as a core hardware supplier for the next generation of AI infrastructure. Under the deal, OpenAI will deploy up to 6 gigawatts of AMD Instinct GPUs across multiple hardware generations — starting with an initial 1 gigawatt rollout of MI450 Series GPUs in the second half of 2026. The agreement builds on earlier collaboration involving the MI300X and MI350X platforms and establishes AMD as a strategic compute partner for OpenAI’s massive AI expansion.
The partnership goes beyond a typical supply contract: to align long-term interests, AMD granted OpenAI a warrant for up to 160 million AMD shares, which will vest in tranches as deployment and performance milestones are achieved. Executives from both companies emphasized the scale and strategic importance of the deal — AMD CEO Lisa Su called it a “true win-win” that brings together AMD’s high-performance compute capabilities with OpenAI’s cutting-edge AI models, while OpenAI CEO Sam Altman said it represents a major step toward building the compute capacity needed to realize AI’s full potential. AMD’s CFO added that the agreement could generate “tens of billions of dollars” in revenue and be highly accretive to earnings, reinforcing investor optimism. The announcement cemented AMD’s reputation as a central player in the AI hardware race and propelled its stock to one of its strongest monthly performances in years.
Micron Technology ($MU) +33.74%
The memory-chip maker benefited from reports of a tightening DRAM supply and rising contract prices. Investors also reacted positively to upbeat analyst revisions and Micron’s optimistic commentary about high-bandwidth memory (HBM) demand in data centers. The company has consistently emphasized that demand for its HBM3E and LPDDR products is outpacing supply, with its AI-related memory sales essentially sold out well into 2026. This backdrop of surging demand and constrained supply fueled expectations for higher margins and accelerated profit growth, helping drive MU’s sharp rally through the month.
Adding to the momentum, Micron unveiled the industry’s highest-capacity low-power DRAM module — the 192GB SOCAMM2, built using advanced LPDDR5X technology. This new memory solution, currently sampling with customers, represents a 50% capacity increase over the prior generation while improving power efficiency by more than 20%. The SOCAMM2 module, designed for next-generation AI data centers, reduces time-to-first-token for real-time inference workloads by more than 80% and can significantly cut energy consumption across large-scale clusters. The product builds on Micron’s five-year collaboration with NVIDIA and expands its leadership in power-efficient AI infrastructure. According to Micron, SOCAMM2 delivers roughly two-thirds better energy efficiency than comparable RDIMM solutions while occupying just one-third the physical space — an innovation expected to make it a core component in future AI servers. The launch underscored Micron’s strategic positioning at the center of the AI hardware supply chain, deepening investor confidence that the company’s technology roadmap aligns perfectly with the ongoing AI data-center buildout.
Teradyne ($TER) +32.05%
Teradyne (maker of automated test equipment and industrial robots) rallied on robust earnings and upbeat guidance. In late October Teradyne reported Q3 revenue of ~$769 million (+4% YoY) and adjusted EPS of $0.85, comfortably above consensus. Management said demand was surging, remarking that the semiconductor test market is entering “a period of accelerating demand driven by advances in AI, semiconductors, and industrial automation.” Teradyne also raised its Q4 outlook (guiding EPS $1.20–$1.46) and analysts promptly lifted price targets. Even a CFO transition announced on Oct. 28 (promoting Michelle Turner to support the growth ramp) was framed positively. The combination of better-than-expected results and strong AI-led outlook sent Teradyne shares up sharply.
Worst S&P 500 Performers in October
On the other end of the market, Fiserv (FISV), Alexandria Real Estate Equities (ARE), and F5, Inc. (FFIV) were the biggest laggards in September. Each grappled with company-specific setbacks that sent their share prices sharply lower.
Fiserv ($FI) –48.27%
Fiserv’s stock tumbled nearly 50% in October following a disappointing third-quarter earnings report that revealed weaker-than-expected growth and a sharp reset to forward guidance. The payments and financial technology provider reported just 1% organic revenue growth in the quarter, as its Merchant Solutions segment expanded 5% while Financial Solutions declined 3%. Adjusted earnings per share fell 11% year over year to $2.04, missing analyst estimates by a wide margin. Management also revised its 2025 outlook downward, now expecting only 3.5% to 4% organic revenue growth and $8.50–$8.60 in adjusted EPS, compared to prior expectations near $10.20. The report highlighted margin compression across both core business units and softer client spending among financial institutions, signaling operational headwinds despite steady payment volume growth in Clover and other merchant products.
In response, the company launched its “One Fiserv” action plan, designed to refocus strategy and restore investor confidence. The initiative centers on five pillars: improving client relationships, scaling the Clover small-business platform, expanding into embedded finance and stablecoin services, leveraging AI for operational efficiency, and maintaining disciplined capital allocation. Leadership changes accompanied the plan — including the appointment of Paul Todd (former CFO of Global Payments) as new CFO, and Takis Georgakopoulos and Dhivya Suryadevara as Co-Presidents effective December 1. While Fiserv emphasized that its fundamentals remain solid and that it retains scale advantages as “the world’s largest Fintech,” CEO Mike Lyons acknowledged performance had fallen short of expectations. He framed the restructuring as a necessary reset to position Fiserv for “sustainable, high-quality growth.” Investors, however, reacted harshly to the guidance cut and management turnover, viewing the announcement as confirmation that growth across key payment segments has stalled — resulting in one of the steepest single-month declines among S&P 500 companies.
Alexandria Real Estate Equities ($ARE) –30.14%
The life-science REIT sold off after a disappointing Q3 update. Alexandria reported declining revenues and funds-from-operations (FFO) year-over-year, and its property occupancy fell to ~91.4% from 94.8% a year ago. Crucially, management cut its 2025 adjusted FFO guidance to $9.01 (from $9.26 prior). The news – coming amid concerns of oversupply in biotech real estate – spurred an analyst downgrade on Oct. 30. In sum, weaker fundamentals (lower FFO, rising vacancies) and a reduced outlook drove Alexandria’s stock down sharply.
F5 Inc. ($FFIV) –21.70%
F5 shares dropped more than 20% in October after the company confirmed a significant cybersecurity breach that overshadowed otherwise solid quarterly results. The network and application security provider reported 8.5% year-over-year revenue growth to around $810 million, exceeding expectations, but its announcement of a long-term intrusion into internal systems rattled investors. According to the company, a sophisticated nation-state threat actor maintained unauthorized access to portions of F5’s BIG-IP product development and engineering knowledge platforms, where it obtained some source code and internal vulnerability information. While F5 said it found no evidence of software supply-chain compromise or active exploitation of its products, the incident sparked broad concern among enterprise customers who rely on its technologies to secure mission-critical workloads.
This article is for informational purposes only and is not investment advice or a solicitation to buy or sell securities. The content is based on publicly available information and reflects the author’s opinions as of the publication date, which may change without notice. All investments carry inherent risks, including the potential loss of principal, and past performance is not indicative of future results. Readers should conduct their own research or consult a financial advisor before making investment decisions. BBAE holds no position in the securities mentioned, nor are they compensated by the companies mentioned.
