Despite evidence supporting the presence of cognitive deficits in children and adolescents with high-functioning autism spectrum disorder (HF-ASD), the nature of these deficits and their clinical and adaptive correlates remain unclear. Moreover, there are few cognitive studies of ASD siblings as a high risk population. We compared 50 children and adolescents with HF-ASD, 22 unaffected siblings of the HF-ASD sample and 34 community controls using an extensive neuropsychological battery. Planning, cognitive flexibility, verbal and working memory, visual local-global processing and emotion recognition are impaired in HF-ASD. Worse cognitive performance, especially in verbal and working memory, was significantly correlated with more severe symptoms and poorer adaptive functioning, also when controlling for intelligence quotient. Results in siblings may suggest an intermediate profile.