Two studies were conducted to examine executive function skills in siblings of children with autism. In Study 1, four computerised tasks (three executive tasks: the ID/ED set-shifting task; a spatial working memory task; and the Tower of London planning task; and a control spatial span task) from the CANTAB battery were used to compare 31 siblings of children with autism with 32 siblings of children with developmental delay and 32 children from unaffected families. In Study 2, the two sibling groups were compared on two manually administered executive tasks (verbal fluency and list recall). As a group, autism siblings showed superior spatial and verbal span, but a greater than expected number performed poorly on the set-shifting, planning, and verbal fluency tasks. There were no group differences in working memory performance. The implications of these findings for the broader phenotype of autism is discussed.