Autism spectrum disorder (ASD) is a heritable disorder occurring predominantly in males. The aim of this study was to compare sex differences in the prevalence of specific neurological phenotypes commonly described in ASD. The study included 663 participants, aged 18 months to 15 years, diagnosed with ASD. Neurological and behavioral assessments were performed using standardized tests, and obtaining medical, developmental, and familial histories from the parents. Phenotypes under investigation were macro- and microcephaly, developmental regression, minor neurological and musculoskeletal deficits (MNMD), and seizures. Male : female ratio in the ASD group was 6.7:1. No sex differences in autism severity, cognitive ability, and adaptive functioning were noted. Mean head circumference percentile for males (50.1 ± 25.6) was significantly larger than females (43.4 ± 30.2). Micro- and macrocephaly were more frequent in ASD than expected (5.9%; 18.1%, respectively). Microcephaly in females (15.1%) was significantly more prevalent than in males (4.5%). The prevalence of macrocephaly in both sexes did not differ significantly. Regression was noted in 30.2% of the females with ASD, significantly higher than in males (18.9%). MNMD was documented in 73.8% of the females, significantly higher than in males (57.1%). M:F ratio decreased in a group with two or more phenotypes (3.6:1), while male predominance was more significant in the group without phenotypes (13.6:1). Neurological phenotypes associated with ASD are more prevalent in females than in males, resulting in more complex clinical and neurological manifestations in females. Therefore, involvement of different etiologies is suggested in ASD in females. Autism Res 2013, 6: 596–604. © 2013 International Society for Autism Research, Wiley Periodicals, Inc.