The effects of methylphenidate on four boys diagnosed as attention-deficit hyperactivity disorder (ADHD) and Tourette's syndrome (TS) were examined under single-blind, placebo-controlled conditions. Clinical ratings and playroom observations showed improvement in ADHD symptoms with methylphenidate. Results also indicated that methylphenidate had no untoward effects on the frequency of tic occurrence. In all four children, the highest dose resulted in improved classroom ratings of tics compared with initial placebo treatment. In three cases, mild tic exacerbation was reported for a lower dose. Because variability of tic status was observed in the experimental conditions, the findings suggest the possibility that tic response was independent of clinical doses of methylphenidate. The findings were also consistent with the theory that methylphenidate, a dopamine agonist, might effect tic status by altering dopamine receptor sensitivity. Further investigation of these effects is indicated, given the efficacy of methylphenidate in treating ADHD symptoms of TS patients.