Twelve patients with abnormal involuntary movement disorders were treated with clozapine in a double-blind, placebo-controlled trial. The cohort consisted of individuals with Gilles de la Tourette's syndrome, Huntington's disease, and atypical persistent dyskinesia that was drug induced. Two subjects were dropped from the protocol due to complications. Two patients with Huntington's disease showed a marked decrease in movements; other individuals obtained no significant therapeutic benefits. Seven of the 10 patients completing the trial experienced moderate or marked side effects.