Background— The endocardial substrate for ventricular arrhythmias in patients with arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) is thought to be caused by a progressive degenerative process. Many clinical decisions and treatment plans are guided by this pathophysiologic assumption, but the extent of progression of macroscopic endocardial scar and right ventricular (RV) dilatation have not been assessed. Methods and Results— Eleven patients with ARVD/C and ventricular tachycardia had 2 detailed sinus rhythm electroanatomic endocardial voltage maps (average, 291±122 points per map; range, 114 to 558 points) performed a mean of 57 months apart (minimum, 9 months) as part of ventricular tachycardia ablation procedures. Voltage-defined scar (<1.5 mV) and RV volume were measured by area and volume measurement software and compared. Two of the 11 patients had a clear increase in scar area (47 cm ² ; 32 cm ² ) confirmed by visual inspection. The remaining 9 (81%; 95% CI, 48% to 98%) patients had no increase (<10-cm ² difference) in scar area between studies. In contrast, 10 of the 11 patients had a significant increase in RV volume, with an average increase of 24% (212±67 mL to 263±52 mL; P ≤0.01). Conclusions— In patients with ARVD/C and ventricular tachycardia, progressive RV dilatation is the rule, and rapid progression of significant macroscopic endocardial scar occurs in only a subset of patients. These results have important management implications, suggesting that efforts to prevent RV dilatation in this population are needed and that an aggressive substrate-based ablation strategy offers the potential to provide long-term ventricular tachycardia control.