The Mahaim syndrome is a rare variant of accessory pathways. Typically, a normal surface ECG and paroxysmal tachycardias with left bundle branch block morphology are found. The anatomic correlate is an atrioventricular or atriofascicular fiber with decremental conduction properties. Criteria for intracardiac mapping of the atrial insertion of such a fiber include: 1) maximal preexcitation and a short stimulus-QRS interval with differential pacing from the atrial aspect of the tricuspid annulus, and 2) recording of an accessory pathway potential ("M" potential). We report a case of a patient that underwent successful radiofrequency ablation of a Mahaim fiber at the tricuspid annulus.