Background: Intra-atrial reentrant tachycardia occurs frequently after surgery for congenital heart disease and is difficult to treat. We tested the hypotheses that intra-atrial reentrant tachycardia in patients who had undergone prior reparative surgery for congenital heart disease could be successfully ablated by targeting a protected isthmus of conduction bounded by natural and surgically created barriers and that entrainment techniques could be used to identify these zones. Methods and Results: Eighteen consecutive patients with 26 intra-atrial reentrant tachycardias complicating surgery for congenital heart disease (9 atrial septal defect repair, 4 Fontan, 2 Mustard, 2 Senning, and 1 Rastelli procedure) underwent electrophysiological study and ablation attempts. Mapping of activation was facilitated by the deployment of catheters with multiple electrodes. Sites for ablation were sought that demonstrated entrainment with concealed fusion and at which the postpacing interval minus the tachycardia cycle length and the stimulus to P wave minus the activation time <30 ms. These sites were considered to be within a narrow isthmus critical to the tachycardia mechanism. Anatomic barriers bordering the critical isthmus of conduction were identified on anatomic grounds by the presence of areas of electrical silence or by the demonstration of split potentials signifying a line of block. Success was achieved in 15 patients with 21 arrhythmias. The median number of radiofrequency application was 5. There was a wide range of activation times at successful sites (-30 to -250 ms). At a mean duration of follow-up of 17±8 months, 11 patients were asymptomatic and 9 did not require antiarrhythmia therapy. Conclusions: Successful ablation of intra- atrial reentrant tachycardia complicating surgery for congenital heart disease may be achieved by creation of an ablative lesion in a critical isthmus of conduction bounded by anatomic barriers. This isthmus may be identified by the presence of entrainment with concealed fusion and an analysis of the relationship between the postpacing interval and the tachycardia cycle length and between the activation time and the stimulus time. Because this isthmus is invariably confined on at least one aspect by a surgical repair site that is of central importance to the tachycardia mechanism, we suggest that this type of arrhythmia be given the descriptive designation of 'incisional reentry.'