Bundle branch re-entrant tachycardia is rarely seen in the electrophysiology lab. Patients who are susceptible to this arrhythmia will generally present with a baseline incomplete left or right bundle branch block (LBBB or RBBB) on electrocardiogram. The most common underlying cardiac disease associated with bundle branch re-entry (BBR) is dilated cardiomyopathy of either ischemic or idiopathic etiology.1 Distinguishing characteristics. The characteristics of BBR that distinguish it from other forms of ventricular tachycardia are tachycardia cycle length of less than 300 ms, and an increase in H-V interval from baseline. Another distinguishing characteristic is that any change in V-V interval is preceded by a change in H-H interval. This tells us that the His-Purkinje system is driving the tachycardia. In non-BBR ventricular tachycardias, changes in V-V interval are followed by changes in H-H interval. In the normal heart, the electrophysiologic characteristics of the His-Purkinje system (rapid conduction and long refractory periods) preclude the formation of a re-entrant pathway.2 Patients with a conduction delay in either bundle branch may slow the electrical impulse enough to allow activation of the opposite bundle branch in a retrograde fashion, thereby creating a re-entrant circuit. Discussion. While catheter ablation of either the right or left bundle branch will result in cessation of the tachycardia, it is less complicated to ablate the right bundle branch than it is to ablate the left. In order to ensure proper catheter position on the right bundle, first locate a His bundle potential on the ablation catheter and then advance the catheter along the His-Purkinje axis until the atrial electrogram becomes small or non-existent.4 While this patient did not meet the usual clinical criteria for BBR (i.e., dilated cardiomyopathy or other structural abnormality), the conduction delay down the right bundle branch was significant enough to create a re-entrant circuit. Although the initiation of transient complete right bundle branch block during the first procedure prevented the induction of tachycardia, and thereby delayed treatment, it was clinically useful in making the correct diagnosis in this patient. Without this valuable clue, it may have been impossible to differentiate this tachycardia from a non-ablatable ventricular tachycardia. Although bundle branch re-entry is seldom seen, it is important to be able to recognize its characteristics, so that appropriate therapy can be delivered. At the time of this writing, the patient is once again leading an extremely active life as a soldier in the United States Army. Because the appropriate diagnosis was made, the implantation of a cardioverter/defibrillator was avoided, to the great relief of the patient, and cost savings that may have exceeded $30,000 for the initial implantation of a cardioverter-defibrillator.