Based on data from the five sites of the National Institute of Mental Health-sponsored Epidemiologic Catchment Area (ECA) Program, this paper examines the prevalence of psychiatric disorder among recent medical service users versus nonusers, with a particular focus on affective disorders, substance abuse/dependence, and phobias. The rate of current Diagnostic Interview Schedule (DIS) disorders among medical users in all five ECA sites is 21.7 per cent (slightly higher than general population rates) versus 16.7 per cent among nonusers; there is generally no difference between users and nonusers with past DIS diagnoses. Affective disorders were among the most common mental disorders of medical service users, especially among females, with little variation between sites: females: users: 6.9 per cent to 9.3 per cent, nonusers: 3.4 per cent to 6.4 per cent, and males: users: 3.3 per cent to 6.5 per cent, nonusers: 1.2 per cent to 4.1 per cent. Rates of phobias among persons using medical services are also higher than among nonusers. Substance abuse disorders are at least as common among persons who use medical services (8 per cent to 14 per cent of male users) as among those who do not (9 per cent to 11 per cent of male nonusers). The high rates of affective disorders among women and of substance abuse among male medical service users underscore the need to increase the ability of general medical practitioners to recognize and manage or refer these conditions.