Background ; In some patients with implantable cardioverter defibrillator (ICD), multiple episodes of appropriate ICD shock can occur, a condition known as electrical storm (ES). Limited data about ES in patients with ICD are available. So, we sought to assess the prevalence, features, and predictors of ES in patients with ICD. Methods ; Consecutive 85 patients with ICD implanted from Oct 1999 to Apr 2009 were retrospectively analyzed. ES was defined as the occurrence of two or more ventricular arrhythmias within 24 hours leading to an immediate electrical therapy (antitachycardia pacing and/or shock), separated by a period of sinus rhythm. Results ; Eighty five patients with ICD were followed for a mean of 861 짹 78 days. A total of 26 patients experienced at least one ES episode. Overall, 115 ES episodes were recorded and 58% patients of the ES group experienced two or more ES episodes. The mean duration for the first ES occurrence after ICD implantation was 209 짹 277 days. In a majority of ES cases, the recorded index arrhythmia was ventricular tachycardia (VT) (65%). There were no obvious etiologic factors at onset of ES episodes (57%). Patients with structurally normal heart (P = 0.043) or ventricular fibrillation (VF) as an index arrhythmia for ICD implantation (P = 0.017) appeared to be more frequent in the ES-free group. Kaplan-Meier estimate and log-rank test identified patients with non-ischemic dilated cardiomyopathy (DCMP) as underlying heart disease (log-rank test, P = 0.016) or with Left ventricular ejection fraction < 35% (log-rank test, P = 0.032) were likely to experience ES. Using Cox proportional hazard regression analysis, non-ischemic DCMP as underlying heart disease was correlated with a greater probability of ES (Hazard ratio = 3.71, 95% Confidence Interval = 1.16 - 11.85, P = 0.027). Conclusions ; ES is common and recurrent event in patients with ICD. Non-ischemic DCMP as underlying heart disease is independent predictor of ES and patients with VF as an index arrhythmia or structurally normal heart are less affected by ES.