A study is reported in which size sequencing on a touch screen is used as a measure of executive control in 20 high-functioning children with Autistic Spectrum Disorders (ASD). The data show a significant and age-independent effect of the length of sequence that can be executed without errors by these children, in comparison with a chronologically age-matched group of children with normal development. Error data and reaction times are analysed and are interpreted as revealing a constraint on the prospective component of working memory in children on the autistic spectrum even when there is no change in goal or perceptual set. It is concluded that the size sequencing paradigm is an effective measure of executive difficulties associated with autism.