To determine shock occurrence and survival, 49 patients with idiopathic dilated cardiomyopathy presenting with cardiac arrest (82%), syncope (12%) or ventricular tachycardia without syncope (6%) were followed for 28 +/- 28 months after cardioverter-defibrillator (ICD) implant according to the intention to treat principle. Using the Kaplan-Meier method, the actuarial incidence for any spontaneous shocks was 20%, 58%, and 77%, and the incidence of appropriate shocks was 16%, 49%, and 72% at 1, 3, and 5 years of follow-up, respectively. Only two of 49 study patients (4%) with an active ICD died suddenly during follow-up. Another two patients, however, with an inactive device died suddenly, resulting in a sudden death rate of 2% per year with an active ICD, and 5% per year, according to the intention to treat principle. The incidence of cardiac death from any cause was 8%, 25%, and 35%, and the incidence of total mortality was 14%, 39%, and 49% during 1, 3, and 5 years of follow-up, respectively. There was no difference in the Kaplan-Meier survival curves for shocked vs non-shocked patients. Thus, in this selected patient population with idiopathic dilated cardiomyopathy the majority of patients received 'appropriate' shocks during follow-up, and the sudden death rate with active ICD is low.