January 2026 is wrapping up, and it's been a month of extremes. A handful of U.S. stocks just posted the best or worst monthly performances of their entire trading histories, driven by everything from blowout earnings to AI-induced panic selling. Let's break down who won, who lost, and why.

The Winners: Record Gains Across Defense, Biotech, and Memory

Start with the good news. Sandisk Corp. (SNDK) had a January to remember, surging more than 150% as the company delivered quarterly earnings that absolutely crushed Wall Street's expectations. The rally wasn't just about one good quarter though. Memory and storage supply is getting tight, thanks to booming AI data center demand that continues to strain capacity across the industry.

For Sandisk, January marked the best-performing month since its IPO back in February 2025. The stock is now up more than 1,400% since going public and claimed the title of top-performing S&P 500 constituent for the month. Not bad for a company that's barely a year old on public markets.

Cameco Corp. (CCJ) advanced 37%, riding a wave of analyst upgrades as expectations for long-term nuclear demand strengthened. Rising uranium prices and renewed policy support for nuclear energy have reinforced the company's growth outlook, making it one of the month's standout performers.

Defense giant Lockheed Martin Corp. (LMT) jumped 30%, logging the strongest monthly gain in the company's history since it began trading in 1995. Here's the interesting part: Lockheed actually missed quarterly earnings estimates, albeit narrowly. But investors didn't care. They focused instead on upbeat 2026 financial guidance and the announcement of a new missile contract with the U.S. Department of Defense. Sometimes the forward-looking story matters more than last quarter's numbers.

Drone maker Karman Holdings (KRMN) also posted its best month ever, climbing 45% after announcing an agreement to acquire Seemann Composites and Materials Sciences. The deal is expected to expand Karman's exposure to the maritime market and strengthen its position within defense supply chains. Defense has been having a moment, and Karman is capitalizing on it.

But the most extreme move of all came from biotech name ImmunityBio Inc. (IBRX), which surged more than 200% to log the strongest monthly performance in the Russell 2000. The rally followed multiple positive updates from clinical-stage cancer treatment trials. When biotech trials go well, the market rewards them aggressively.

The Losers: Software Gets Crushed by AI Fears

Now for the painful side. January's worst performers were overwhelmingly software stocks, as artificial intelligence continues to reshape demand across the sector and force investors to reassess business models. This wasn't about one or two companies stumbling. This was a sector-wide repricing event.

Braze Inc. (BRZE) plunged 37%, while HubSpot Inc. (HUBS) fell 30%, Rubrik Inc. (RBRK) dropped 27%, Guidewire Software Inc. (GWRE) slid 28%, and GoDaddy Inc. (GDDY) declined 21%. All five names logged their worst monthly performances on record. And here's what makes it particularly brutal: the selloff was largely unrelated to company-specific earnings releases.

Instead, the pressure reflected a broader sector-wide repricing. The software rout intensified Thursday after Microsoft Corp. (MSFT) issued weaker-than-expected guidance for its Azure cloud business, triggering a 10% collapse in its shares. That marked Microsoft's worst single-day drop since March 2020, when pandemic panic was at its peak.

The shock reverberated across the entire sector. When Microsoft sneezes, software stocks catch pneumonia. The iShares Expanded Tech-Software Sector ETF (IGV) fell 14% for the month, marking its steepest monthly decline since October 2008, at the onset of the financial crisis. Yes, you read that right: the worst month for software since Lehman Brothers collapsed.

What's driving this? AI disruption is forcing investors to ask hard questions about which software companies will thrive in an AI-native world and which ones are building products that AI might eventually replace or commoditize. That uncertainty is showing up in valuations, and it's not pretty.

What This Tells Us About Markets Right Now

January's extreme moves reveal a market in the middle of massive sector rotation. Money is flowing toward companies that benefit from AI infrastructure buildout—think memory suppliers like Sandisk—and toward defensive plays like Lockheed Martin that offer stability in uncertain times. Meanwhile, software companies that thrived in the pre-AI era are getting hammered as investors question their competitive moats.

Biotech remains as volatile as ever, with ImmunityBio (IBRX) proving that positive clinical trial data can still generate explosive returns. And nuclear energy, represented by Cameco (CCJ), continues to attract attention as AI data centers demand more baseload power.

The lesson? In a market being reshaped by AI, the winners and losers are being decided quickly and decisively. January gave us a preview of what that looks like when it happens at warp speed.