A Tic Checklist and direct observation tic measurement procedure were developed for the assessment of Tourette syndrome in individuals with mental retardation. Using a single-subject reversal design, we applied this assessment method to the evaluation of haloperidol treatment for a subject with Tourette syndrome and severe mental retardation. Relative to baseline, haloperidol 10 mg/day produced decreases of 66% in simple motor tics, 46% in complex motor tics, 45% in simple vocal tics, and 50% in complex vocal tics. Improvement was also seen in careprovider ratings of tic severity, hyperactivity, and compulsive behaviors.