Evidence for self-selection among health maintenance organization enrollees.

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.