Managed care may influence technology diffusion in health care. This article empirically examines the relationship between HMO market share and the diffusion of neonatal intensive care units. Higher HMO market share is associated with slower adoption of mid-level units, but not with adoption of the most advanced high-level units. Opposite the common supposition that slowing technology growth will harm patients, results suggest that health outcomes for seriously ill newborns are better in higher-level units and that reduced availability of mid-level units may increase their chance of receiving care in a high-level center, so that slower mid-level growth could have benefitted patients.