Abstract Background Primary prevention implantable cardioverter defibrillator (ICD) therapy is indicated in patients with a LVEF<35%. Increasing evidence suggests that LVEF is a poor predictor or ventricular arrhythmias, such as ventricular tachycardia (VT) or fibrillation (VF). Methods Patients hospitalized for VF (55.4%) or VT with hemodynamic compromise underwent late gadolinium enhancement cardiovascular magnetic resonance (CMR). Patients with congenital heart disease, channelopathies, hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy and chemotherapy-induced cardiomyopathy were excluded. Results Among patients (n=111, aged 62.1 ± 14.6 years [mean ± SD]) hospitalized for VF (55.4%) or VT with hemodynamic compromise (44.5%) only 29/111 (43.6%) had a history of cardiac disease. At the index hospitalization, an ischemic cause was identified in 69.3%, a non-ischemic cause in 24.8% and no cause emerged in 5.9%. The LVEF (39.1 ± 17.2%) was ≤35% in 55.4% and >35% in 44.6%. Myocardial fibrosis was present in 85.1% (LVEF≤35%: 91.1%; LVEF>35%: 77.8%; p = 0.0619). Conclusions A large proportion of sustained VT or VF survivors would not have previously met LVEF primary prevention criteria for ICD therapy. In this 'ICD treatment gap', myocardial fibrosis was more prevalent than a LVEF≤35%. Further studies are needed to determine whether myocardial scar rather than LVEF≤35% should be used as an indication for primary prevention ICD therapy.