This study examines the mix of cases in facilities for the treatment of end-stage renal disease in Michigan during the period January 1973 through September 1981. We compared 3135 patients treated in 29 hospital-based facilities with 307 patients treated in five proprietary, free-standing facilities. Patients were assigned to one of five severity groups on the basis of age, race, primary renal diagnosis, and accompanying conditions. The five severity groups were differentiated by the probability of death in the first year of treatment and the risk of death over the course of treatment. We then compared the distribution of patients in the five severity groups in hospital-based facilities with that in free-standing facilities. Hospital-based facilities had a higher percentage of patients in the higher-severity groups. When severity was measured by one-year survival, the difference was statistically significant. Sixty per cent of hospital-based patients were in the three highest severity groups, as compared with 50 per cent of patients in free-standing facilities. Within each severity group, hospital-based patients had a lower five-year survival rate than patients in free-standing facilities. Our findings suggest that the case mix in hospital-based facilities may include more severe cases than that in proprietary, free-standing facilities, but more data from more facilities will be needed before firm conclusions can be drawn. If the cost of providing services is related to case-mix severity, such data could have implications for federal reimbursement policies.