The diffusion of health care technology is influenced by both the total market share of managed care organizations as well as the level of competition among them. This paper differentiates between HMO penetration and competition and examines their relationship to the adoption of cardiac catheterization laboratories in all non-federal, short-term general community hospitals in the U.S. between 1985 and 1995. Results show that a hospital is less likely to adopt the technology if HMO market penetration increases but more likely to adopt if HMO competition increases. Further, the latter effect is non-linear in the number of adopters. In markets where fewer than a critical number of neighboring hospitals have already adopted, the probability of adoption increases with the number of HMOs, but in markets where more than the critical number of neighbors have already adopted, the probability of adoption decreases with the number of HMOs. Thus, in markets where technology is rare, HMO penetration and competition have countervailing effects on the diffusion of technology such that the net effect could be small.