Bullying is a significant problem among school-age children. The prevalence and predictors of bullying among children with autism are not known. The objectives of this population-based study were to: (1) estimate the prevalence of bullying among children with autism in the United States, (2) determine whether the presence of attention-deficit/hyperactivity disorder/attention-deficit disorder (ADHD/ADD) increases prevalence of bullying among children with autism, and (3) determine risk factors of bullying behavior among children with autism. The National Survey of Children's Health, 2003 (NSCH), provided nationally representative data for children ages 4 to 17. We used multivariate logistic regression and Wald tests to determine whether children with autism were more likely to bully in the presence of ADHD/ADD. Taylor approximations were used to account for the complex sampling design. Children with autism had a high prevalence of bullying (44%, 95% confidence interval, 34-55). Parent report of ADHD/ADD appears to moderate the relationship between bullying and autism. Children with autism who did not have ADHD/ADD were not at greater risk for bullying compared with the general population. Children with autism and ADHD/ADD had increased odds of bullying (odds ratio 4.6, 95% confidence interval 2.4-8.6), even after controlling for household income, age, and gender. In addition to ADHD/ADD, living in a low-income household and younger age were risk factors for bullying among children with autism. Being female, however, did not decrease the risk of bullying in the autistic subpopulation. Children with autism and ADHD/ADD appear to be at increased risk for bullying behaviors.