Although autism spectrum disorders (ASD) prevalence is higher in males than females, few studies address sex differences in developmental functioning or clinical manifestations. Participants in this study of sex differences in developmental profiles and clinical symptoms were 22 girls and 68 boys with ASD (mean age = 28 months). All children achieved strongest performance in visual reception and fine motor followed by gross motor and language functioning. Sex differences emerged in developmental profiles. Controlling for language, girls achieved higher visual reception scores than boys; boys attained higher language and motor scores and higher social-competence ratings than girls, particularly when controlling for visual reception. Longitudinal, representative studies are needed to elucidate the developmental and etiological significance of the observed sex differences.