Aggression is a clinically significant problem for many children and adolescents with autism spectrum disorders (ASD). However, there have been few large-scale studies addressing this issue. The current study examined the prevalence and correlates of physical aggression in a sample of 1584 children and adolescents with ASD enrolled in the Autism Treatment Network. The prevalence of aggression was 53%, with highest prevalence among young children. Aggression was significantly associated with a number of clinical features, including self-injury, sleep problems, sensory problems, GI problems, communication and social functioning. In multivariate models, self-injury, sleep problems, and sensory problems were most strongly associated with aggression. The results indicate that aggression is markedly prevalent, and clinical implications and directions for future research are discussed. (Contains 3 tables and 1 figure.)