Objective: The purpose of this study was to examine the effect of maternal depression during the child's first year of life (i.e., sensitive period) on subsequent behavior problems. Method: Participants were 175 mothers participating in the Oregon Adolescent Depression Project (OADP) who met lifetime diagnostic criteria for major depressive disorder (MDD) and completed the child behavior checklist (CBCL) for their first child at some point during the child's first 12 years (mean = 4.91 years). Results: Regression analyses indicated that MDD in the sensitive period was a significant predictor of internalizing and total behavior problems on the CBCL while controlling for several demographic variables (e.g., child and mother age, child gender). Maternal depression before pregnancy and during the prenatal period did not significantly predict later child behavior problems, suggesting that the effect was not driven by the presence of previous MDD and was specific to the first year of life. Conclusions: Presence of maternal MDD during a child's first year of life represents a sensitive period and increases the risk of adverse child outcome. The findings suggest the importance of identification, prevention, and early intervention. Future studies should examine these findings in more diverse, heterogeneous samples. (Contains 2 tables.)