This study examined the effects of beta blockers on (1) appropriate implantable cardioverter defibrillator (ICD) therapy for ventricular tachycardia (VT) or ventricular fibrillation (VF), (2) inappropriate ICD therapy for atrial fibrillation or supraventricular tachycardia, and (3) survival in 691 patients who received ICDs in the Multicenter Automatic Defibrillator Implantation Trial-II. The study population involved 258 patients who were not receiving beta blockers and 433 who were receiving metoprolol (n = 192), atenolol (n = 58), or carvedilol (n = 182). Patients receiving beta blockers were divided into the upper quartile and lower 3 quartiles of the drug doses they were taking. Patients receiving the higher doses of beta blockers (those in the top quartile of doses) had a significant reduction in the risk for VT or VF requiring ICD therapy compared with patients not receiving beta blockers (hazard ratio 0.48, p = 0.02). The frequency of inappropriate ICD therapy for supraventricular tachyarrhythmias was not significantly different among the 3 treatment groups (p = 0.32). Beta-blocker use at the 2 dosage levels was associated with significant improvement in survival compared with the nonuse of beta blockers (hazard ratios 0.42 to 0.44, p <0.01). In conclusion, beta blockers reduce the risk for VT or VF and improve survival in ICD-treated patients with ischemic cardiomyopathy.