This DataWatch evaluates four-stage models of market evolution developed initially by the University HealthSystem Consortium (UHC). Such models suggest that increasing health maintenance organization (HMO) penetration is linked with increases in hospital consolidation and vertical integration between physicians and hospitals. These claims are tested using national data for 1992-1995. Results suggest that such models accurately classify the markets of UHC member hospitals according to their levels of HMO penetration only. Moreover, they do not discern evolutionary stages of market development and may not be generalizable to the markets of non-UHC member hospitals. Researchers and policymakers should exercise caution in applying such models.