Objective: This study examined childhood behavior problems at ages 10 and 11 years as predictors of young adult depression, social phobia, and violence at age 21 years. Method: Data were collected as part of the Seattle Social Development Project, a longitudinal study of 808 elementary school students from high-crime neighborhoods of Seattle. Reports of childhood behavior problems were obtained from parents and children in fall 1985 and from teachers in spring 1986. Follow-up reports of violence and DSM-III-R depression and social phobia were collected from 765 respondents using standard survey items and the Diagnostic Interview Schedule in 1996. Results: The past-year prevalences of depressive episode and social phobia were 20% and 17%, respectively. Twenty-one percent of the respondents reported two or more violent acts in the past year at age 21 years. Several available measures of childhood behavior problems as reported by parents, teachers, and children predicted violence (e.g., conduct problems, oppositional defiance); the strongest positive predictor of young adult violence was self-reported conduct problems, whereas self-reported shyness inhibited later violence. Relatively few child behavioral problems predicted social phobia (e.g., shyness). Results showed that children who reported higher, relative to lower, levels of conduct problems were nearly four times more likely to experience a depressive episode in early adulthood. Conclusions: Findings suggest the potential value of intervening to reduce childhood conduct problems as a prevention strategy for not only violence but also depression.