This paper estimates the effect of market structure on hospital days and ambulatory visits in independent practice associations (IPAs) and group-model health maintenance organizations (HMOs) where market structure is measured by HMO penetration and the number of HMOs operating in a market. There was a steady decline in inpatient use in HMOs during the study period and a steady increase in use of ambulatory care. In multivariate analyses, inpatient use is significantly higher in IPAs, but there is no difference in ambulatory use. As HMO penetration increases and the number of HMOs increases, group-model HMOs have lower hospital use and greater ambulatory use. In contrast, use of both inpatient and ambulatory care decreases in IPAs but only at high levels of penetration and numbers of competitors.