Objective: This study aimed to explore sex differences in autistic traits in relation to diagnosis, to elucidate factors that might differentially impact whether girls versus boys meet diagnostic criteria for autism or a related autism spectrum disorder (ASD). Method: Data from a large population-based sample of children were examined. Girls and boys (aged 10-12 years) meeting diagnostic criteria for an ASD were compared with those failing to meet diagnostic criteria despite very high scores on a trait measure of ASD, the Childhood Autism Spectrum Test (CAST). Information about behavioral difficulties as reported by teachers, and early estimates of intellectual functioning, were compared. Results: Girls, but not boys, meeting diagnostic criteria for ASD showed significantly more additional problems (low intellectual level, behavioral difficulties) than peers with similarly high CAST scores who did not meet diagnostic criteria. Conclusions: These data suggest that, in the absence of additional intellectual or behavioral problems, girls are less likely than boys to meet diagnostic criteria for ASD at equivalently high levels of autistic-like traits. This might reflect gender bias in diagnosis or genuinely better adaptation/compensation in girls. (Contains 2 tables and 4 figures.)