Despite sophisticated atrial and ventricular pacing techniques used during electrophysiologic study, the exact mechanism of some tachycardias remain elusive. In this situation, eliminating conduction from specific tissue and observing the effect on the tachycardia can be diagnostic. Such a maneuver can be performed using the technique of ice mapping, which entails reversible cooling of tissue to test function prior to delivery of a permanent ablation lesion. We present a case of wide-complex tachycardia with inconclusive results from diagnostic electrophysiologic study. Using ice mapping, cooling at the region of the compact AV node resulted in block in the ascending fast pathway. When cooling subsequently was applied to the region of the slow pathway, tachycardia broke in the descending limb; thus AV nodal reentrant tachycardia was diagnosed on an anatomic and functional basis.