This study examined the long-term outcomes of mental health treatment, specifically individual therapy, group therapy, and pharmacotherapy, in a population-based study. Using a prospective cohort design, the effectiveness of mental health treatment was analyzed on the basis of data from an epidemiological catchment area follow-up study, which assessed health care use and mental health status at 2 times, 15 years apart, in a random sample of Baltimore residents. A cohort of 771 men and women with at least 1 diagnosable Diagnostic and Statistical Manual of Mental Disorders disorder was selected from the 1,920 individuals randomly sampled in 1980 who were followed in 1994-1996. The results are consistent with clinical trials on the efficacy of pharmacotherapy and psychotherapy. After controlling for baseline psychopathology, sociodemographic characteristics, and service providers, participants who received group and individual therapy had lower distress at follow-up than those who either received medication or did not seek or receive any treatment. No concomitant reductions in the number of disorders were found, except for a dose-response association between the number of therapy sessions and the number of disorders at follow-up, as well as distress at follow-up.