Clonidine has been suggested to be effective in Gilles de la Tourette's syndrome (GTS), but no double-blind study has ever evaluated its effects using objective measures. Thirty patients with GTS completed a 6-month placebo-controlled crossover study of the effectiveness of clonidine. Videotapes were obtained at each 3-week visit and were evaluated randomly at the end of the study for distribution, frequency, and severity of motor and vocal tics. Quantifiable psychometric examinations were performed as well. The use of clonidine did not significantly (p < 0.05) reduce motor tics, vocalizations, or behavior. The effect of a low dose (0.0075 mg/kg/day) was no different from that of a high dose (0.015 mg/kg/day); children's responses were no different from adults'; and those also receiving neuroleptic agents showed the same lack of efficacy as seen in patients on no other medication. Dosing schedule did not affect the objective ratings; scores from clonidine given twice a day were equivalent to those for three times a day.