IBM's mainframe business, a cornerstone of its profitability for decades, suffered a 42% revenue decline in the latest quarter, but the company's leadership insists the drop is a temporary side effect of the AI boom, not a sign of the platform's obsolescence. The shocking miss triggered a 25% stock crash, the worst single-day drop in the company's history, and forced CEO Arvind Krishna to issue an unprecedented pre-earnings warning to investors. Main Developments IBM reported $17.2 billion in revenue and $2.2 billion in net earnings for the quarter, with gross profit margins near 58%. Despite those healthy numbers, the results fell well short of Wall Street expectations, prompting Krishna to publish a letter to investors last week sharing preliminary results that warned of abysmal revenue in the infrastructure category and a hit to profit margins. The company's stock immediately plummeted 25%, erasing gains that had been buoyed by the AI data center boom during Krishna's six-year tenure. IBM also lowered its full-year growth forecasts, signaling that the bad quarter would drag on the remainder of the year. Read also: Google's Cloud Surge Justifies Its AI Spending Spree The core problem: mainframe hardware sales dropped 42%. CFO Jim Kavanaugh explained on the quarterly earnings call that for every dollar of mainframe hardware sold, IBM earns three dollars in software revenue, making the hardware decline a cascading financial blow. Background Mainframes have been IBM's cash cow for generations, powering critical systems for banks, airlines, and large enterprises. Each system costs hundreds of thousands to millions of dollars, and the accompanying maintenance contracts and software licenses generate many millions more over the lifecycle. The tech industry has predicted the death of the mainframe for decades, yet the platform has consistently survived new waves of technology, from client-server computing to cloud. This quarter's drop, however, is directly tied to the AI build-out that has reshaped data center spending across the industry. Krishna noted that dozens of customers who were due to buy a new mainframe instead diverted their budgets to other hardware. Enterprise hardware makers like Dell and HP have warned that rising costs on components such as memory — driven by the AI boom — have forced them to raise prices by 15% to 30%. Apple has issued similar warnings. Why It Matters The mainframe decline threatens IBM's most profitable revenue stream. With software revenue tied directly to hardware sales, a sustained drop would erode the high-margin business that underpins the company's financial stability. The 42% plunge suggests that AI infrastructure spending is cannibalizing traditional enterprise IT budgets, a shift that could reshape the competitive landscape. If other enterprise hardware vendors face similar budget reallocations, the AI boom may prove to be a double-edged sword: lifting some companies while undermining the legacy products that have long sustained them. For IBM, the question is whether the mainframe can survive yet another technological disruption, or whether this is the beginning of a structural decline. What's Next Krishna assured investors that customers who delayed mainframe purchases this quarter have already begun buying them in the current quarter, calling the slowdown a temporary blip. He stated, "We see no evidence of clients moving off the mainframe." The company will need to demonstrate a recovery in mainframe sales in the coming quarters to restore investor confidence. IBM's lowered full-year forecasts mean the company faces pressure to show that the mainframe business can rebound even as AI continues to drive data center cost increases. Analysts and investors will watch closely for signs that the delayed purchases materialize — or whether the AI boom permanently shifts enterprise spending away from legacy systems.