Tardive dyskinesia, a syndrome of involuntary motor movements, can be a permanent consequence of the long-term use of antipsychotic drugs. While there is no well-established drug treatment, case reports and the results of a few clinical studies suggest that drugs that facilitate the GABA-ergic system may decrease the abnormal movements. One such class of drugs is the benzodiazepines. We administered diazepam to 13 subjects in a 24-week, crossover design study. Tardive dyskinesia and psychopathology were assessed by blind raters using the Abnormal Involuntary Movement Scale and the Brief Psychiatric Rating Scale (BPRS). The means of all movement measurements improved from the baseline, with orofacial, subtotal, symptom severity, and total reaching significance. However, we were unable to demonstrate a drug effect; the patients improved to a similar degree whether or not they received diazepam. Their psychiatric disorders did not worsen with diazepam administration and, in fact, improved slightly; the activation factor of the BPRS was significantly improved over baseline. Our results suggest that diazepam is not effective in managing the movements of tardive dyskinesia and that behavior modification strategies be investigated to help patients control symptoms.