BACKGROUND This randomized trial evaluated an integrated model of primary medical care for a cohort of patients with serious mental disorders. METHODS A total of 120 individuals enrolled in a Veterans Affairs (VA) mental health clinic were randomized to receive primary medical care through an integrated care initiative located in the mental health clinic (n = 59) or through the VA general medicine clinic (n = 61). Veterans who obtained care in the integrated care clinic received on-site primary care and case management that emphasized preventive medical care, patient education, and close collaboration with mental health providers to improve access to and continuity of care. Analyses compared health process (use of medical services, quality of care, and satisfaction) and outcomes (health and mental health status and costs) between the groups in the year after randomization. RESULTS Patients treated in the integrated care clinic were significantly more likely to have made a primary care visit and had a greater mean number of primary care visits than those in the usual care group. They were more likely to have received 15 of the 17 preventive measures outlined in clinical practice guidelines. Patients assigned to the integrated care clinic had a significantly greater improvement in health as measured by the physical component summary score of the 36-Item Short-Form Health Survey than patients assigned to the general medicine clinic (4.7 points vs -0.3 points, P<.001). There were no significant differences between the 2 groups in any of the measures of mental health symptoms or in total health care costs. CONCLUSIONS On-site, integrated primary care was associated with improved quality and outcomes of medical care.