Research in school-aged children, adolescents, and adults with autism spectrum disorder (ASD) has found sex-based differences in behavioral, developmental, and diagnostic outcomes. These findings have not been consistently replicated in preschool-aged children. We examined sex-based differences in a large sample of 2-5-year-old children with ASD symptoms in a multi-site community-based study. Based on a comprehensive evaluation, children were classified as having ASD (n = 1480, 81.55 % male) or subthreshold ASD characteristics (n = 593, 70.15 % male). Outcomes were behavior problems, developmental abilities, performance on ASD screening and diagnostic tests, and parent-reported developmental conditions diagnosed before study enrollment. We found no statistically significant sex differences in behavioral functioning, developmental functioning, performance on an ASD screening test, and developmental conditions diagnosed before study enrollment among children with ASD or subthreshold ASD characteristics. Males in both study groups had more parent reported restricted interests and repetitive behaviors than females, but these differences were small in magnitude and not clinically meaningful. Preschool males and females who showed risk for ASD were more similar than different in the outcomes assessed in our study. Future research could examine sex-based differences in ASD phenotypes as children age.