Four hundred twenty consecutive patients with symptomatic slow/fast atrioventricular nodal reentry tachycardia had attempted slow pathway radiofrequency ablation. All patients had successful slow pathway ablation and no inducible tachycardia after ablation using the standard right-sided approach except for three patients. The three unsuccessful patients had inducible slow/fast atrioventricular nodal tachycardia after attempted right-sided posterior and inferoposterior anatomic ablative techniques and with slow pathway potential electrogram guidance. Lesions were also delivered linearly in the triangle of Koch and within the coronary sinus ostium. A transseptal puncture was performed and slow pathway ablation was obtained in each of these patients. Ablation was performed from the septal mitral valve annulus, anterior to the os of the coronary sinus and inferior to the His-bundle catheter with elimination of slow pathway conduction. Prior to the ablation, two of the three patients exhibited initiation of tachycardia with a double fast/slow antegrade response, and all three patients had long AH intervals (mean 378 ms) during slow pathway conduction. These electrophysiological findings may be consistent with a large area of slow pathway conduction that may include the left atrial septum not approachable by conventional right-sided ablative techniques. Slow pathway ablation to eliminate atrioventricular nodal reentry tachycardia at times may require a left atrial/transseptal approach when conventional right-sided techniques are ineffective.