Parents and teachers reported that 6- to 8-year-old boys with developmental delays were less able to regulate their emotions than nondelayed boys matched on chronological age. Compared to nondelayed boys, boys with developmental delays had more social problems, which persisted and increased over a 3-year period. Children's ability to regulate their emotions explained significant variance in their social problems after controlling for their developmental status. In addition, emotion regulation partially mediated the relationship between children's developmental status and their social problems. These results suggest that emotion regulation plays a significant role in the social problems of boys with developmental delays. Furthermore, increasing the emotional competence of these children may facilitate their peer relationships and, ultimately, their school adjustment.