Using a blind evaluation of cinematographic films of patients suffering from tardive dyskinesia we found that thiopropazate hydrochloride in a dosage up to 30 mg daily was effective in reducing the severity of the dyskinesia in most patients. The overall improvement in the group of patients studied was not significant after one or three months of therapy but was significant after six months of treatment. The administration of thiopropazate hydrochloride over a six month period did not appear to aggravate the underlying pathophysiology so that the drug could be considered likely to be safe for long-term use. The anticholinergic antiparkinsonism agent benztropine mesylate aggravated the dyskinesia to a significant degree.