Mental health data were gathered at ages 11, 13, 15, 18, and 21 in an epidemiological sample using standardized diagnostic assessments. Prevalence of Diagnostic and Statistical Manual of Mental Disorders (3rd ed. revised; American Psychiatric Association, 1987) mental disorders increased longitudinally from late childhood (18%) through mid-(22%) to late-adolescence (41%) and young adulthood (40%). Nearly half of age-21 cases had comorbid diagnoses; and comorbidity was associated with severity of impairment. The incidence of cases with adult onset was only 10.6%: 73.8% of adults diagnosed at age 21 had a developmental history of mental disorder. Relative to new cases, those with developmental histories were more severely impaired and more likely to have comorbid diagnoses. The high prevalence rate and significant impairment associated with a diagnosis of mental disorder suggests that treatment resources need to target the young adult sector of the population. The low new-case incidence in young adulthood, however, suggests that primary prevention and etiological research efforts need to target children and adolescents.