Health care experience among employees prior to and after enrollment in a prepaid health insurance plan (HMO).

Company medical and benefit records of employees who were enrolled in prepaid health insurance plans (HMOs) in Minneapolis, Washington, D.C., and Seattle were reviewed for hospital and surgical utilization prior to as well as after their enrollment. Comparisons were made for the same calendar periods with closely matched employees who were covered by Blue Cross/Blue Shield (BC/BS). The results indicate that after enrollment the HMO subjects at each location had lower rates for hospital and surgical utilization than the BC/BS controls. For the period prior to enrollment, during which time coverage was through BC/BS, the prospective HMO subjects at two locations had lower hospital and surgical utilization than the controls. Possible explanations of this potential self-selection bias are discussed.