This study analyzes the effectiveness of the implantable cardioverter defibrillator in a group of 40 consecutive patients with idiopathic dilated cardiomyopathy and sustained ventricular arrhythmia; these patients were compared with a control group of 80 consecutive patients with proved coronary artery disease. There were no differences in the type and probability of recurrences at follow-up. Most recurrences were due to sustained monomorphic ventricular tachycardia, and the efficacy of the defibrillator in treating the arrhythmia was the same in both groups.