This study examined sleep problems and their correlates and comorbid psychopathology in children with autism spectrum disorders (ASDs). Participants consisted of 166 ASD children and 111 unaffected siblings aged 4-15 years. Parents completed a self-administered child sleep questionnaire. Of the children with ASDs, 47.0% (78/166) had at least one sleep problem. Children with ASDs were more likely to have bedtime resistance (OR 2.72, 95% CI 1.02-7.21), insomnia (OR 4.07, 95% CI 1.60-10.35), and daytime sleepiness (OR 7.62, 95% CI 1.17-49.38) compared to their unaffected siblings. Among children with ASDs, multivariate logistic regression analyses indicated that communication abnormalities and repetitive stereotyped behaviors were associated with increased risk of sleep problems. ASD children with sleep problems were more likely to have withdrawal problems, somatizing problems, aggressive behaviors, internalizing problems, externalizing problems, and total behavioral problems compared to those without sleep problems. Sleep problems are prevalent in children with ASDs and are associated with both internalizing and externalizing problems. Clinicians should pay close attention to sleep problems when assessing and treating autistic behaviors. (Contains 4 tables.)