Increasing managed care activity could influence the adoption and diffusion of new medical technologies. This paper empirically examines the relationship between HMO market share and the diffusion of magnetic resonance imaging (MRI) equipment. Across markets, increases in HMO market share are associated with slower diffusion of MRI into hospitals between 1983 and 1993, and with substantially lower overall MRI availability in and outside of hospitals in the mid and later 1990s. High managed care areas also had markedly lower rates of MRI procedure use. These results suggest that technology adoption in health care can respond to changes in financial and other incentives associated with managed care, which may have implications for health care costs and patient welfare.