Current guidelines have indicated an implantable cardioverter-defibrillator (ICD) for patients with severe idiopathic dilated cardiomyopathy, for both primary and secondary prevention. Compared to coronary artery disease, the overall benefit has been smaller. A more refined risk assessment, using the left ventricular ejection fraction (LVEF) and prevention mode (primary/secondary), is still needed to guide ICD implantation. Patients included in 2 large ICD registers were analyzed regarding the appropriate therapies and improvement of LVEF, overall and in subgroups of prevention mode and LVEF < 20% versus > 20%. Overall, 349 patients were included; 70% were men, the mean age was 54 years, and the mean follow-up was 33 months. Cardiac resynchronization therapy (CRT) was used in 57%, and secondary prevention was present in 30%. ICD therapies were delivered to 33% of the patients, in most for ventricular tachycardia. Patients receiving an ICD for secondary prevention and non-CRT were more likely to have arrhythmic events (both p < 0.05). The cumulative event rates at 5 years were 53% for secondary and 33% for primary prevention (p < 0.001). Depending on the prevention mode and LVEF status (< 20% vs > 20%), the event rates ranged from 30% to 76%. The mean LVEF improved by 10%, independently of the stimulation mode (CRT 22% to 31%, non-CRT 26% to 35%; p < 0.0001). A persistent improvement to > 35% was seen in only 25% of CRT patients but in 45% of non-CRT patients (p = 0.004). In conclusion, the results from the present study have demonstrated that in patients with idiopathic dilated cardiomyopathy, the potential for LVEF improvement is considerable and that the rate of ICD interventions strongly depends on the prevention mode and LVEF. These findings could be the basis for additional risk stratification tools.