The authors compared the effects of age, sex, socioeconomic status, and mental and physical health status on the use of outpatient mental health services in one well-established health maintenance organization (HMO) and in fee-for-service plans. In the Seattle site of the Rand Health Insurance Study (HIS), families were randomly assigned to HMO or fee-for-service coverage. Adults incur much greater expense for outpatient mental health services than children in both an HMO and a fee-for-service plan with identical coverage (i.e., free care). However, the difference in use between adults and children is significantly greater for the fee-for-service plan than the HMO (P< 0.01). Similarly, education has significantly greater effects on use for the fee-for-service than the HMO plan. Increased income has a significant negative effect on use in both the HMO and fee-for-service plans. Mental and physical health status have similar large effects on use in both fee-for-service and HMO plans.