Patients with implantable cardioverter-defibrillators often receive adjunctive antiarrhythmic therapy to prevent frequent shocks. We tested the efficacy and safety of sotalol, a beta-blocker with class III antiarrhythmic effects, for this purpose. In a multicenter trial, patients were stratified according to left ventricular ejection fraction ( 0.30), randomly assigned to double-blind treatment with 160 to 320 mg of sotalol per day (151 patients) or matching placebo (151 patients), and followed for 12 months. Kaplan-Meier analyses of the time to an event were performed. Three end points were used: the delivery of a first shock for any reason or death from any cause, the first appropriate shock for a ventricular arrhythmia or death from any cause, and the first inappropriate shock for a supraventricular arrhythmia or death from any cause. Compliance with double-blind treatment was similar in the two groups. There were seven deaths in the placebo group and four in the sotalol group. As compared with placebo, treatment with sotalol was associated with a lower risk of death from any cause or the delivery of a first shock for any reason (reduction in risk, 48 percent; P<0.001 by the log-rank test), death from any cause or the delivery of a first appropriate shock (reduction in risk, 44 percent; P=0.007), or death from any cause or the delivery of a first inappropriate shock (reduction in risk, 64 percent; P=0.004). Sotalol also reduced the mean (+/-SD) frequency of shocks due to any cause (1.43+/-3.53 shocks per year, as compared with 3.89+/-10.65 in the placebo group; P=0.008). In the sotalol group, the reduction in the risk of death from any cause or the delivery of a first shock for any reason did not differ significantly between patients with ejection fractions of more than 0.30 and those with ejection fractions of 0.30 or less. Oral sotalol was safe and efficacious in reducing the risk of death or the delivery of a first defibrillator shock whether or not ventricular function was depressed.