Twelve of 60 consecutively studied patients undergoing electrophysiologic study for paroxysmal supraventricular tachycardia had atrioventricular (A-V) bypass tracts functioning as the retrograde limb of the reentrant circuit. None had evidence of preexcitation in the surface electrocardiogram, but in two patients anterograde preexcitation could be produced by pacing from the coronary sinus. In all 12 patients with concealed bypass tracts the retrograde atrial activation sequence or effect of left bundle branch block aberration during the tachycardia, or both, confirmed the left-sided bypass tract. A negative P wave in lead I during the tachycardia was also diagnostic of a left-sided bypass tract. Dual A-V nodal pathways were found in five patients with concealed bypass tracts but were unrelated to the development of the tachycardia. When compared with supraventricular tachycardia due to A-V nodal reentry, clinical findings suggestive of a concealed bypass tract included: (1) P wave following the QRS complex (12 of 12 versus 12 of 40), (2) negative P wave in lead I during the tachycardia, and (3) bundle branch block aberration during the tachycardia (8 of 12 versus 3 of 40). Other characteristics of patients with concealed bypass tracts that were of less value in individual cases were shorter cycle lengths of tachycardia, younger patient age and lesser incidence of organic heart disease.