The occurrence of ventricular arrhythmias in patients with idiopathic dilated cardiomyopathy (DCM) who are treated with an implantable cardioverter-defibrillator (ICD) for primary or secondary prevention is not fully understood. In this nonrandomized, two-centre, observational study we analyzed the occurrence of ventricular arrhythmias in a total of 105 DCM patients (age, 53 +/- 13 years) treated with an ICD. Fifty-one patients with a left ventricular ejection fraction <or= 35% did not have prior sustained ventricular arrhythmias (primary prevention). The secondary prevention group consisted of 54 patients with documented sustained ventricular tachycardia (n = 25) or aborted sudden cardiac death (n = 29). During 32 +/- 7 months follow-up the number of patients with appropriate defibrillator therapies (n = 51) was comparable between the two groups (HR 0.79, 95% CI 0.454 to 1.361, P = 0.389). Importantly, less primary prevention patients experienced appropriate ICD shocks for any arrhythmic event (HR 0.35, 95% CI 0.186 to 0.777, P = 0.008), as well as appropriate ICD shocks for ventricular fibrillation (HR 0.31, 95% CI 0.167 to 0.737, P = 0.006). In contrast, antitachycardia pacing was more often observed in the primary prevention group (HR 2.75, 95% CI 1.031 to 6.238, P = 0.043). Two primary prevention and 6 secondary prevention patients received multiple ICD therapies in consequence of incessant ventricular tachycardia. The characteristics of ventricular arrhythmias in patients with DCM who are treated with an ICD for primary or secondary prevention vary according to the underlying indication. Therefore, different device programming according to the patient's history might improve ventricular tachyarrhythmia management.