Both longitudinal and cross-sectional studies utilizing population and family study samples have found evidence for a secular increase in major affective disorders in adults. Applying techniques used in cross-sectional studies in adults to family study data of children and adolescents, the authors demonstrate evidence of a parallel secular increase for child and adolescent onset affective disorders. Normal and depressed prepubertal probands were identified. All full siblings were directly interviewed for lifetime episodes of affective disorder. Analysis of the siblings (probands not further analyzed in this article) by the Cox proportional hazards model demonstrates that the risk for affective disorder is higher in siblings born more recently.