One-month prevalence results were determined from 18,571 persons interviewed in the first-wave community samples of all five sites that constituted the National Institute of Mental Health Epidemiologic Catchment Area Program. US population estimates, based on combined site data, were that 15.4% of the population 18 years of age and over fulfilled criteria for at least one alcohol, drug abuse, or other mental disorder during the period one month before interview. Higher prevalence rates of most mental disorders were found among younger people (less than age 45 years), with the exception of severe cognitive impairments. Men had higher rates of substance abuse and antisocial personality, whereas women had higher rates of affective, anxiety, and somatization disorders. When restricted to the diagnostic categories covered in international studies based on the Present State Examination, results fell within the range reported for European and Australian studies.