Calcium channel blockers, antiarrhythmic drugs, such as verapamil and diltiazem, may decrease the symptoms of tardive dyskinesia. The efficacy and safety of administering 60 mg diltiazem hydrochloride, four times daily for a period of 3 weeks, was studied in a random, double-blind, crossover trial in which the drug was compared with placebo in 17 neuroleptic-treated, chronic psychiatric inpatients of both genders with (tardive) dyskinesia. The severity of the dyskinesia was assessed using the Abnormal Involuntary Movement Scale. Neither diltiazem nor placebo produced a significant decrease in the severity of the dyskinesia. Diltiazem did not influence the psychiatric state of the patients, nor did it have a significant effect on either the blood pressure or electrocardiographic parameters. No significant adverse drug reactions were elicited.