BACKGROUND: Parietal block, defined as intra right ventricular (RV) conduction slowing, is a major diagnostic criterion for arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C). OBJECTIVE: We evaluated the utility of total RV endocardial activation duration (EAD) measured by 3-dimensional electroanatomic mapping during sinus rhythm in the diagnosis of ARVD/C. METHODS: Twenty-five consecutive patients with frequent left bundle branch block morphology premature ventricular complexes who underwent electroanatomic mapping as a part of the evaluation for ARVD/C were included in the study. All patients were evaluated using standard protocol that included electrocardiogram (ECG), signal-averaged ECG, Holter monitoring, echocardiography, and magnetic resonance imaging. Invasive testing was performed as indicated. Total RV EAD was measured as the time interval between the onset of RV activation to the latest activated region in the RV. RESULTS: The mean age of the study subjects was 38 +/- 11 years, and 32% were men. Fourteen subjects were diagnosed with ARVD/C using task force criteria, and the remainder had idiopathic ventricular tachycardia. Although the surface QRS durations were similar, the total RV EAD was significantly prolonged in ARVD/C compared with idiopathic VT (83.9 +/- 10 ms vs. 50.8 +/- 7 ms, P <.001). None of the idiopathic VT subjects had RV EAD of >65 ms. RV EAD also showed significant negative correlation with RV ejection fraction. CONCLUSION: Total RV EAD obtained by 3-dimensional electroanatomic mapping is a sensitive marker of intra-RV conduction delay in ARVD/C, and a total RV EAD of >65 ms accurately differentiates ARVD/C from idiopathic VT.