OBJECTIVE: The aim of this study was to examine patterns of use of general medical services among persons with a severe and persistent mental illness enrolled in Medicaid from 1996 to 1998. METHODS: A total of 669 persons with a severe and persistent mental illness were identified by using statewide clinical criteria. A three-year database of Medicaid claims was developed to examine service use. The main outcome measures were use of outpatient services for a general medical problem, use of dental and vision services, and use of screening tests for women. Service use was examined by primary psychiatric diagnosis (schizophrenic, affective, paranoid, and anxiety disorders), and analyses controlled for the presence of a chronic medical condition, age, race, and sex. RESULTS: This study found high levels of service use for outpatient services but very low levels for primary and preventive services. Although 78 percent of persons with a schizophrenic disorder had an office-based visit during the three-year period, all persons with an anxiety disorder had such a visit. Sixty-nine percent of persons with a schizophrenic disorder had at least one emergency department visit, whereas 83 percent of those with an anxiety disorder had such a visit. Dental and vision visits and the use of mammograms and pap tests followed the same pattern; persons with a schizophrenic disorder had fewer visits and had less overall use than the other diagnostic groups. The use patterns across the four groups were significantly different in outpatient service use, dental and vision service use, and screening tests for women. Compared with persons with a schizophrenic disorder, those with an anxiety disorder were more likely to have had an office-based visit and to have received vision services, those with a paranoid disorder were more likely to have used dental services or received a mammogram, and those with an affective disorder were more likely to have had a pap test. CONCLUSION: Although this group of Medicaid patients with severe and persistent mental illness had access to providers, they received an unacceptably low level of preventive care. Use of health services for general medical problems differed somewhat by primary psychiatric illness.