Proarrhythmia is relatively common after extensive atrial ablation for atrial fibrillation, but has not been frequently documented after catheter ablation of ventricular tachycardia (VT). In theory, this phenomenon could occur if an incomplete ablation line is created between two nonconducting structures, such as dense scar or valvular annuli. This report illustrates the possible proarrhythmic effect of ablation in a patient with sarcoid-related VT and extensive right ventricular (RV) myopathy who presented with slow incessant VT one month after an ablation procedure including ablation at the pulmotricuspid isthmus (PTI). The extensive preexisting RV myopathy appeared to be an important substrate in the pathogenesis of this patient's incessant VT. This case suggests that the PTI region may serve as a critical tachycardia isthmus if sufficiently modified with an incomplete ablation line in the setting of significant myocardial scarring.