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OBJECTIVE: We compare mental health utilization in prepaid and fee-for-service plans and analyze selection biases. DATA SOURCE: Primary data were collected every six months over a two-year interval for a panel of depressed patients participating in the Medical Outcomes Study, an observational study of adults in competing systems of care in three urban areas (Boston, Chicago, and Los Angeles). STUDY DESIGN: Patients visiting a participating clinician at baseline were screened for depression, followed by a telephone interview, which included the depression section of the NIMH Diagnostic Interview Schedule. Patients with current or past lifetime depressive disorder and those with depressed mood and three other lifetime symptoms were eligible for this analysis. We analyze mental health utilization based on periodic patient self-report. ANALYTIC METHODS: We use two-part models because of the presence of both nonuse and skewness of use. Standard errors are corrected nonparametrically for correlations across observations due to clustered sampling within participating physicians and repeated observations on the same individual. PRINCIPAL FINDINGS: The average number of mental health visits was 35-40 percent lower in the prepaid system, adjusted and unadjusted for observed differences in patient characteristics, including health status. Utilization differences were concentrated among patients of psychiatrists, with only minor differences among patients of general medical providers. Analyzing the effect of switches that patients make between payment systems over time, we found some evidence of adverse selection into fee-for-service plans based on baseline utilization, but not based on utilization at the end of the study. In particular, after adjusting for observed patient characteristics and health status, patients switching out of prepaid plans had higher baseline use than predicted, whereas patients switching out of fee-for-service had lower use than predicted. Switching itself appears to be related to an immediate decline in utilization and was not followed by an increase or "catch-up" effect. CONCLUSIONS: The absence of the commonly found "catch-up" effect following switching and the significant decrease in utilization during the switching period suggests an interruption in care that does not occur for patients staying within a payment system. This finding emphasizes the need for integrating new patients quickly into a system, an issue that should not be neglected in the current policy discussion.