We found that preschoolers with ASD demonstrated a higher prevalence of sleep problems than preschoolers in the community. Specifically, to the best of our knowledge, this is the first study to suggest that obstructive sleep apnea is more common in preschoolers with ASD than in community preschoolers, although the reason remains to be clarified. The present study also showed that sleep problems, especially insomnia, are related to problematic behaviors in preschoolers with ASD. These results highlight the importance of routine assessment and treatment of sleep problems that should greatly help children with ASD and their families.