This monograph reviews and assesses twenty-five studies that examined the question of whether treatment for mental illness, alcohol abuse or drug abuse reduces subsequent medical care utilization. In general, the studies found that such a reduction did take place. Twelve of thirteen studies found reductions of 5 to 85 per cent in medical care utilization subsequent to a mental health intervention. The median reduction was 20 per cent. The thirteenth study found that mental health services provided in a new neighborhood health center in a medically underserved neighborhood were followed by a 72 per cent increase in medical care encounters. The remaining twelve studies found reductions of 26 to 69 per cent in either medical care utilization or surrogate measures of such utilization subsequent to treatment for alcohol abuse. The median reduction was 40 per cent. The drug abuse literature in this area is sparse and primarily indirect. Although many of the studies suggested that alcohol, drug abuse or mental health (ADM) treatment was a cause of the subsequent reduction in medical care utilization, such causality was not definitively established, due to frequent methodological limitations, such as inadequate comparison groups, short time spans, small samples and lack of trend analysis. In addition, the studies focused primarily on outpatient psychotherapy in organized health care settings, particularly health maintenance organizations (HMOs), and on alcoholism treatment provided through employee-based programs and HMOs. Only very limited policy implications on such topics as health insurance and the linkage of health and ADM services can be drawn from the current body of literature. There is a need for additional research that is broader and more rigorous. To encourage such research, methodological recommendations for future studies are presented.