Dear Sir, During antiarrhythmic drug therapy for various kinds of cardiac arrhythmias, not only arrhythmogenic effects, but also other minor adverse reactions, such as skin eruption, dry mouth, and urinary retention, have been recognized as causes of discontinuation and poor compliance of therapy. Recently we observed a case of another rare but significant adverse effect of an antiarrhythmic drug. The patient was a 46-year-old intellectual desk worker with intermittent Wolff-Parkinson-White syndrome. He had occasional episodes of paroxysmal supraventricular tachycardia while playing amateur baseball and drinking alcoholic beverages. We prescribed the long-acting disopyramide phosphate, 150 mg once in the morning daily, because of low efficacy of verapamil, 40 mg three times daily. On the first visit after administration of disopyramide, he complained of sexual dysfunction (impotence) without any other anticholinergic side effects. At the second visit (4 weeks after drug administration), he reported the disappearance of that adverse effect. The steady-state serum level of disopyramide in this case was 1.5 ~g/ml. Sexuality is woven into the everyday life of an individual. Sexual dysfunction is a matter of enormous importance and the cause of worry and concern. Several authors [1-4] have itemized and reviewed the characteristics of good sexual functioning and its dysfunction. Papadopoulos [3] described the significance of the actions of cardiovascular drugs as factors in sexual dysfunction of the cardiovascular patient. McHaffie et al. [2] reported a case of a 47-year-old man in whom impotence developed when the plasma concentration of the drug was 14 ~g/ml. There was full recovery of sexual function when the concentration was lowered to 3 ~g/ml. Although the anticholinergic properties of the drug may mainly impair sexual performance [2-4], the present case indicates the dependence on individual organ sensitivity to the anticholinergic action of the drug. Despite the low serum drug concentration, the adverse drug action happened as a first-dose phenomenon at the beginning of drug administration. This case also indicates that such adverse effects of the long-acting disopyramide at a low serum concentration may be transient and reversible with continued intake of the drug.