The President's Commission on Mental Health has highlighted a heretofore unmet need for the linkage of data on the prevalence of mental disorder with national data on the use of mental health services. This study analyzes available epidemiological data and recent mental health services research findings to estimate the percent of the population with a mental disorder and the proportion utilizing various types of specialty mental health and general medical treatment settings. Provisional estimates indicate that at least 15% of the US population is affected by mental disorders in one year. In 1975, only one fifth of these were served in the specialty mental health sector, with about three fifths identified in the general medical (primary care) sector.