Despite the importance of understanding factors related to physician adoption and use of diagnostic technologies, relatively few studies have been published. Results of a two-year study of the adoption of magnetic resonance imaging (MRI) and its substitution for computed tomography scanning (CT) are presented. The literature on physician adoption and use of technology is used to provide a framework for this study. Differences in adoption and substitution among medical specialties, early versus late adopters, and high versus low users of MRI are examined. Results show that neurologists and internists more rapidly adopt MRI and substitute it for CT than do orthopedists and other surgical specialists. Referral of higher numbers of patients is the best predictor of more rapid substitution. Physicians who were late adopters more quickly substituted MRI for CT. The cost and social implications of empirical versus "ideal" substitution rates are discussed along with how various regulatory, technology assessment, and financial strategies influence substitution. The role of individual physicians, radiologists, and specialty societies in determining substitution rates is also discussed.