Inpatient utilization and health care costs of employees and their families were studied for 11 employee groups covered by Blue Cross and Blue Shield in the Minneapolis-St Paul area during the year before optional health maintenance organization (HMO) enrollment. Before enrolling in HMOs, those joining averaged 53% fewer inpatient days (470 days/1,000 v 994 days/1,000) than those retaining fee-for-service (FFS) coverage. Both hospital and professional expenditures were lower among HMO enrollees. Age-specific analysis showed higher inpatient use of continuing FFS subscribers in every category. Self-selection may be an important determinant of differences in use and costs when comparing HMOs with FFS coverage.