In the absence of intellectual impairment autism spectrum disorder (ASD) is diagnosed both less and later in females. This study used clinician and teacher report to explore sex differences in the behavioural presentation of 69 girls and 69 boys all diagnosed with high-functioning ASD. Evidence from DSM-IV-TR and DSM-5 are presented. Sex differences in teacher concerns were also explored. While no sex differences were found in the broad social criteria presented in the DSM-IV-TR or DSM-5, numerous differences were evident in how boys and girls came to meet each criterion. For example, girls were more likely to show an ability to integrate non-verbal and verbal behaviours, maintain a reciprocal conversation, and be able to initiate, but not maintain friendships. Moreover, girls presented with both less and different restricted interests. Teachers also reported substantially fewer concerns for girls than boys, including for externalising behaviours and social skills. Results suggest girls with ASD may present with a surface-level 'look' different from the 'classic' presentation of ASD, and present as less impaired when in a school setting. Consequently, results provide insight in to why the disorder may be more difficult to detect in cognitively-able girls.