We started 2026 by delivering revenue and adjusted operating income above the high end of our Q1 guidance. Revenue for the first quarter was $310 million, up 1.5% year-over-year, and adjusted operating income margin was 35%, up more than two points year-over-year. While we exceeded our guidance in Q1, as macro conditions worsened at the end of the quarter, we experienced a regression in our downmarket and upmarket growth trajectories. LLMs have given go-to-market teams a simpler interface to work with data and build custom revenue workflows without heavy technical support.
With an increasingly headless approach to software, these workflows become materially more valuable when powered by our data and insights. We are shifting investment from front-end application development toward data, AI-enabled engineering, product-led growth, LLM interfaces, and higher-margin customer segments. While this may create a near-term revenue headwind, it gives us a cleaner model and a better opportunity to grow as customers expand their use of ZoomInfo data. We're confident in our ability to generate strong cash flow and operate the business efficiently while we execute this strategic shift.
We remain committed to returning capital to shareholders in the most value-accretive way possible while ensuring we maintain long-term flexibility. Unlevered free cash flow was $120 million, with $21 million in interest paid in cash during the quarter. In a seasonally slower quarter, upmarket ACV grew 5% year-over-year, a step down from 6% year-over-year growth in the fourth quarter, an improvement from 3% upmarket ACV growth in the year-ago period. As Henry highlighted, Operations had another strong quarter, with ACV growth greater than 20% year-over-year.