OBJECTIVE: The authors examined the volume and predictors of outpatient mental health utilization among primary care patients in a large staff- model health maintenance organization (HMO). METHOD: Consecutive primary care patients (N = 1,810) were screened by using the 12-item General Health Questionnaire, and a stratified random sample (N = 373) completed the 28-item General Health Questionnaire and Composite International Diagnostic Interview. Telephone interviews and computerized records were used to examine use of mental health services inside and outside the HMO over the following 3 months. RESULTS: Over 3 months, 6.7% of the screened patients used mental health services within the HMO. Utilization increased with higher General Health Questionnaire score (2.9% among those scoring 0, 22.3% among those scoring 8 or more) and decreased with higher out-of-pocket cost for mental health visits (7.5% for those with no change, 3.3% for those paying $30/visit). Among the interviewed subjects, 5.1% used mental health services within the HMO (mean = 2.92 visits) and 8.9% used outside mental health services (mean = 8.86 visits). Use of outside services was more strongly related to sociodemographic factors, and use of inside services was more related to severity of psychological disorder. CONCLUSIONS: Among these subjects, use of mental health care was high and services purchased outside the HMO exceeded those inside the HMO. Increasing copayment levels progressively reduced demand without respect to severity of illness. Attempts to control outpatient mental health costs must address equity and clinical need.Abstract Teaser