The Childhood Autism Spectrum Test (CAST) (formally known as the Childhood Asperger Screening Test) identifies autism spectrum conditions by measuring social and communication skills. The present study explored the sex distribution of scores. The CAST was distributed to 11,635 children aged 4-9 years in Cambridgeshire primary schools (UK). 3,370 (29%) were returned. The median score was significantly higher in boys (median test, P less than 0.001) (Boys, median = 5 (IQR: 3,8); girls, median = 4 (IQR: 2,6)). There was a predominance of boys (n = 81 (79.4%) over girls (n = 21 (20.6%)) amongst those scoring greater than or equal to 15. Exclusion of children with ASC did not significantly affect the results. We conclude that different profiles of social and communication skills in boys and girls must be taken into account when measuring these skills in the general population.