This study explores the developmental trajectory of externalizing problems in a sample of 101 children of adolescent mothers from preschool through third grade using hierarchical linear models (HLM). First, a detailed assessment of the structure of the developmental trajectory of externalizing problems is provided. Second, the impact of three risk factors (infant attachment, maternal depressive symptomatology, and child sex) on the developmental course of externalizing problems is assessed. Both avoidant and disorganized attachment and higher levels of maternal depressive symptomatology were associated with higher levels of externalizing problems at 9 years of age. Girls also showed higher externalizing problems relative to their same-sex peers than did boys. In addition, maternal depressive symptomatology related to the rate of change in these problems over time; the greater the mother's depression, the faster externalizing problems tended to increase. Although the overall level of maternal depressive symptomatology was related to children's externalizing problems for secure, avoidant, and disorganized groups, changes in maternal depressive symptomatology over time predicted levels of externalizing problems only for children with avoidant insecure attachments.