Regional differences in hospital utilization are quite substantial, especially between the East Coast and West Coast. To determine whether or not differences in the socioeconomic composition of regional populations explain observed regional differences in hospital utilization, the authors assessed patterns of utilization for a sample of 18,660 New York City residents, 18,881 Los Angeles residents, 10,921 persons living in Northeastern Standard Metropolitan Statistical Areas (SMSAs) and 13,475 persons living in Western SMSAs. The results showed that the New York–Los Angeles difference in hospital days of care decreased from 43% to 22% after adjusting for differences in seven distinct socioeconomic characteristics of the two populations. The Northeastern–Western difference decreased from 18% to -6% (i.e., the Western rate exceeded the Eastern rate after adjustment). Adjusting for differences in socioeconomic characteristics of the local populations affected regional differences in admission rates more than differences in lengths of stay. In the case of New York City, the study also showed that the presence of an unusual number of very long hospital episodes (51 or more days) also is an important explanation of New York's high average stay. The difficulty of identifying posthospital placements for patients needing extended care is thought to cause these long stays in New York City.