OBJECTIVES: Whether beta-blockers reduce atrial arrhythmias and, when added to an angiotensin-converting enzyme (ACE) inhibitor, ventricular arrhythmia is unknown. BACKGROUND: Ventricular and atrial arrhythmias are common after acute myocardial infarction (AMI) and are associated with a poor prognosis. Angiotensin-converting enzyme inhibitors reduce the incidence of both types of arrhythmia. METHODS: The antiarrhythmic effect of carvedilol was examined in a placebo-controlled multicenter trial, the Carvedilol Post-Infarct Survival Control in Left Ventricular Dysfunction (CAPRICORN) study, which enrolled 1,959 patients with reduced left ventricular systolic function after AMI, 98% of whom were treated with an ACE inhibitor. RESULTS: The incidence of atrial fibrillation/flutter was 53 to 984 (5.4%) in the placebo group and 22 to 975 (2.3%) in the carvedilol group, giving a carvedilol/placebo hazard ratio (HR) of 0.41 (95% confidence interval [CI] 0.25 to 0.68; p = 0.0003). The corresponding rates of ventricular tachycardia/flutter/fibrillation were 38 to 984 (3.9%) and 9 to 975 (0.9%) (HR 0.24, 95% CI 0.11 to 0.49; p < 0.0001). CONCLUSIONS: Carvedilol has a powerful antiarrhythmic effect after AMI, even in patients already treated with an ACE inhibitor. Carvedilol suppresses atrial as well as ventricular arrhythmias in these patients.