OBJECTIVE This study attempts to determine the ratio of full-time-equivalent psychiatrists to members and that of nonphysician mental health professionals to psychiatrists in staff and group model health maintenance organizations (HMOs), and to compare the psychiatrist-to-member ratio with previous estimates of the required psychiatrist-to-population ratios in fee-for-service and managed care environments. METHODS The Group Health Association of America (now the American Association of Health Plans) collected data on mental health staffing, enrollments, and other characteristics for 30 staff and group model HMOs. The authors evaluated the number of full-time-equivalent psychiatrists and nonphysician mental health professionals per 100,000 HMO members, and the ratio of full-time-equivalent nonphysician mental health professionals to psychiatrists. RESULTS The overall mean number of full-time-equivalent psychiatrists and nonphysician mental health professionals per 100,000 members in the responding HMOs was 6.8 and 22.9, respectively. The overall mean ratio of nonphysician professionals to psychiatrists was 4.5. The overall number of psychiatrists per 100,000 members is less than half the requirement estimated by the Graduate Medical Education National Advisory Committee in 1980, which assumed a fee-for-service environment, but it is about 40% to 80% greater than that estimated by other studies under the assumption of a managed care environment. CONCLUSIONS Although a practice environment dominated by managed care may not require as high a psychiatrist-to-population ratio as a predominantly fee-for-service environment, it may well support a greater number of psychiatrists than previous studies have suggested.