OBJECTIVE: This study examines two dimensions of racial segregation across hospitals, using a disease for which substantial disparities have been documented. DATA SOURCES: Black (n=32,289) and white (n=244,042) patients 67 years and older admitted for acute myocardial infarction during 2004-2005 in 105 hospital markets were identified from Medicare data. Two measures of segregation were calculated: Dissimilarity (i.e., dissimilar distribution by race across hospitals), and Isolation (i.e., racial isolation within hospitals). For each measure, markets were categorized as having low, medium, or high segregation. STUDY DESIGN: The relationship of hospital segregation to residential segregation and other market characteristics was evaluated. Cox proportional hazards regression was used to evaluate disparities in the use of revascularization within 90 days by segregation level. RESULTS: Agreement of segregation category based on Dissimilarity and Isolation was poor (kappa=0.12), and the relationship of disparities in revascularization to segregation differed by measure. The hazard of revascularization for black relative to white patients was lowest (i.e., greatest disparity) in markets with low Dissimilarity, but it was unrelated to Isolation. CONCLUSIONS: Significant racial segregation across hospitals exists in many U.S. markets, although the magnitude and relationship to disparities depends on definition. Dissimilar distribution of race across hospitals may reflect divergent cultural preferences, social norms, and patient assessments of provider cultural competence, which ultimately impact utilization.