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Background: It has been established that areas of slow conduction within a myocardial scar identified by isochronal mapping during sinus rhythm harbor the functional substrate that is involved in sustaining ventricular tachycardia (VT). We sought to test the hypothesis that targeting the region of slowest conduction during sinus rhythm would reduce VT recurrence following ablation. Methods and Results: 32 subjects underwent ablation for sustained monomorphic VT associated with structural heart disease from 2013 to 2014. Sustained VT recurred in 12 patients (37.5%). Isochronal late activation maps were created to display activation during sinus rhythm in the region of bipolar scar. The scar was divided into three zones of equal activation time. The zone with the densest isochrones was designated as having the slowest conduction . We retrospectively analyzed isochronal maps and measured the proportion of the slowest zone that was ablated (median 14%, IQR 0-50). During a mean follow-up of 6 months, recurrence of ventricular arrhythmia was significantly associated with ablation of the slowest zone (OR 0.126, CI 0.024-0.68, p 0.016). Furthermore, univariate logistic regression demonstrated reduction of 30% in the 6-month VT recurrence rate for every 10% increase in percent of the slowest zone ablated (OR 0.7, 95% CI 0.5-1.0, p=0.05). Conclusions: Patients who had ablation in the region of slowest conduction were significantly less likely to have recurrence of ventricular tachycardia. Our data suggests a strategy to target the slowest region of conduction for substrate modification may hold promise for improving outcomes of scar-mediated VT ablation.