Objective: No longitudinal studies beginning in childhood have estimated the cumulative prevalence of psychiatric illness from childhood into young adulthood. The objective of this study was to estimate the cumulative prevalence of psychiatric disorders by young adulthood and to assess how inclusion of not otherwise specified diagnoses affects cumulative prevalence estimates. Method: The prospective, population-based Great Smoky Mountains Study assessed 1,420 participants up to nine times from 9 through 21 years of age from 11 counties in the southeastern United States. Common psychiatric disorders were assessed in childhood and adolescence (ages 9 to 16 years) with the Child and Adolescent Psychiatric Assessment and in young adulthood (ages 19 and 21 years) with the Young Adult Psychiatric Assessment. Cumulative prevalence estimates were derived from multiple imputed datasets. Results: By 21 years of age, 61.1% of participants had met criteria for a well-specified psychiatric disorder. An additional 21.4% had met criteria for a not otherwise specified disorder only, increasing the total cumulative prevalence for any disorder to 82.5%. Male subjects had higher rates of substance and disruptive behavior disorders compared with female subjects; therefore, they were more likely to meet criteria for a well-specified disorder (67.8% vs 56.7%) or any disorder (89.1% vs 77.8%). Children with a not otherwise specified disorder only were at increased risk for a well-specified young adult disorder compared with children with no disorder in childhood. Conclusions: Only a small percentage of young people meet criteria for a "DSM" disorder at any given time, but most do by young adulthood. As with other medical illness, psychiatric illness is a nearly universal experience. (Contains 2 tables and 3 figures.)