• Gilles de la Tourette's syndrome was independently ascertained in two male cousins once removed. Previous studies have shown familial clustering of individuals with tics, but no consistent pattern of inheritance of Tourette's syndrome has been apparent. The onset and later exacerbation of symptoms in our younger patient were associated with the administration of CNS stimulants given for excessive motor activity. The adverse effects of methylphenidate and dextroamphetamine therapy on Tourette's syndrome supports the hypothesis that this condition results from a relative excess of CNS catecholaminergic activity. Physicians prescribing these agents should inquire about the presence of tics in patients and their families.