Objective: To examine the development of first-onset major depressive disorder (MDD) in children at high and low familial risk for depression in a prospective study. Method: High-risk children (n = 76) who were free of any lifetime affective disorder and had at least one first-degree and one second-degree relative with a lifetime history of childhood-onset, recurrent, bipolar, or psychotic depression were included. Low-risk children (n = 63) were included if they were free of any lifetime psychiatric disorder and had no first-degree relatives and fewer than 20% of their second-degree relatives with a lifetime affective disorder. Children and their parents were assessed in a prospective design using the Schedule for Affective Disorders and Schizophrenia for School-Age Children-Epidemiologic version (K-SADS-E). The average interval between follow-up interviews was 18 months, and the average follow-up period was 6 years. Results: High-risk children had approximately a threefold increased risk of developing first-onset MDD compared with low-risk children (odds ratio = 3.21). The average age of new-onset MDD was 14.0 [+ or -] 2.9 years (range 9.5-19.5 years). Above and beyond the familial loading for MDD, mother's lifetime anxiety disorder (odds ratio = 2.84) and lifetime behavioral disorder (odds ratio = 3.25) in the child significantly added to the risk of developing a first-onset MDD. Conclusions: Having high familial loading for affective disorders, a mother with and anxiety disorder, and a behavioral disorder in the child all significantly contributed to the risk of developing depression.