The work is based on an analysis of a clinical case in a patient with paroxysmal tachycardia with narrow QRS complexes, which was diagnosed as WPW phenomenon by electrocardiography. A differential electrophysiological diagnosis was performed. It was proven that tachycardia with narrow complexes was an atypical form of atrioventricular nodal reciprocal tachycardia, and the WPW phenomenon is a concomitant diagnosis. This finding allowed to perform a successful radiofrequency ablation of tachycardia without redundant applications and X-ray exposure. The obtained data allow to clarify the existing knowledge about paroxysmal tachycardia and to improve the results of diagnosis and surgical treatment of this category of patients.