This analysis of out-of-plan use of physician and paramedical services under a Kaiser plan and under a prepaid option offered by a predominantly fee-for-service group practice (Clinic plan) deals primarily with services that members could have obtained from plan providers ("covered services"). The extent and pattern of out-of-plan use were found to be similar. While 16-20 per cent of plan members used some out-of-plan covered services and the mean number of such services was about one half visit per member per year, most out-of-plan user were occasional user, 10-12 per cent of user (or 2 per cent of plan members) accounting for 50 per cent of all out-of-plan covered services. The principal members characteristics associated with out-of-plan use were dissatisfaction, health status and having other insurance. The literature on out-of-plan use is also reviewed.