Rising competitive pressures may place uninsured patients at risk for receiving fewer services than insured patients with similar medical conditions. To examine this possibility we studied the case mix, length of stay, and number of procedures for 65 032 patients listed as self-pay or free care, Blue Cross, or Medicaid at 52 hospitals in the Boston, Mass, area during 1983. We found that the overall case mix severity index (based on expected length of stay per diagnosis related group) for uninsured patients was 30% higher in public hospitals and 8% higher in major teaching hospitals compared with other institutions. Across all hospitals, the severity index of uninsured patients was similar to that of insured patients. However, after adjusting for diagnosis related group case mix, uninsured patients had, on average, 7% shorter stays (5.36 vs 5.79 days) and underwent 7% fewer procedures (1.16 vs 1.25) than Blue Cross patients, the differences varying with hospital type. Uninsured patients also had shorter stays on average than Medicaid patients (5.36 vs 5.87 days), but they underwent a similar number of procedures. These results suggest that patients who lack insurance may receive unequal treatment even after being hospitalized.