This study examined the nature of cognitive and behavioral sex differences in children with autism spectrum disorders (ASDs) and two comparison groups: a group of typically developing (TD) children and a group of unaffected siblings of ASD children. Sex differences in core autistic symptoms, co-occurring behavioral symptoms, and cognitive styles were assessed in each group. Females with ASD were less severely affected than IQ- and age-matched males with ASD in the communication and repetitive stereotyped behavior domains, as measured by the ADI-R, but such sex differences were less significant than they were in the unaffected sibling group. Several behavioral/emotional symptom scores were significantly higher in male siblings than in female siblings. However, the ASD and TD groups did not show sex differences in any behavioral/emotional symptom scores. Males were superior in systemizing relative to empathizing, while the opposite was true for females in the unaffected sibling group and the TD children group; however, both males and females were superior in systemizing relative to empathizing in the ASD group. Our findings support the extreme male brain theory of autism, and further studies are needed to understand the mechanisms behind and developmental perspectives on the nature of sex differences in ASD. (Contains 4 tables.)