We performed a randomized, double-blind, placebo-controled, parallel-group comparison study of the efficacy and safety of sotalol in the prophylaxis of paroxysmal supraventricular tachyarrhythmias (PSVTs) (including paroxysmal atrial fibrillation and paroxysmal reentrant SVTs). The frequency of PSVT episodes while not receiving drug therapy was monitored during a baseline phase, the length of which depended on the frequency of PSVT events. In the double-blind phase, the duration of which depended on the baseline frequency of episodes of PSVT, patients received placebo, sotalol 80 mg twice daily, or sotalol 160 mg twice daily. PSVT events were documented by electrocardiogram and diary. The time to recurrence of PSVT was significantly less compared with placebo when receiving sotalol 80 mg ( p = 0.018) and sotalol 160 mg ( p = 0.0009). On subanalysis, sotalol was shown to be effective in the prophylaxis of both paroxysmal atrial fibrillation and paroxysmal reentrant arrhythmias. Sotalol was well tolerated, with no deaths, proarrhythmia, or cardiac failure. Because of adverse effects, drug therapy was discontinued in six patients. (Am Heart J 1997;133:441-46.)