Previous research has suggested that there may be dysfunction in the control of the hypothalamic-pituitary-adrenal axis in autistic children. Both an abnormal cortisol circadian rhythm and failure to suppress cortisol secretion in response to dexamethasone have been reported. This study investigated the basal urinary cortisol circadian rhythm in a group of high-functioning children with autism and matched controls. No evidence was found for abnormal temporal placement of the circadian rhythm in the autistic group. There was a tendency towards cortisol hypersecretion during the day, predominantly in those autistic children who were integrated into the normal school system. While the temporal parameters of the cortisol circadian rhythm in these children with autism were probably normal, the tendency towards cortisol hypersecretion may indicate an environmental stress response in this group.