Recently there have been proposed electrophysiologic criteria for the diagnosis of a concealed atrio-His bypass tract in patients with paroxysmal supraventricular tachycardia (PST). In order to verify the reliability of the proposed criteria--among the patients we studied for PST without ventricular preexcitation and in whom retrograde His bundle activation was recorded--we have chosen those with normal anterograde A-V conduction, with constant (or minimal increases) V-A interval during ventricular pacing and with short (less than 50 msec) and constant H2-A2 interval during ventricular premature stimulation. We encountered 15 patients with these electrophysiologic characteristics (37-73 years). H2-A2 interval (measured from the end of the His bundle deflection to the earliest atrial activity) ranged 20-45 msec (mean: 33 msec). The prolongation of S2-A2 interval observed in all patients was always within the S2-H2 tract. The retrograde effective refractory period of the A-V nodal region was always short, but not evaluable in any of the patients since it was shorter than that of ventricular myocardium (14 cases) or of His-Purkinje system (1 case). The S1-H2 interval was measured to evaluate whether during ventricular premature stimulation the retrograde His bundle activation was in fact anticipated. In 14 out of 15 patients this interval varied within a very narrow range: 0-20 msec. For this reason we believe that the differential diagnosis between a concealed atrio-His bypass and an accelerated retrograde A-V conduction can only be made if during electrophysiologic study a tachycardia is induced; in such case the detection of an H-Ae interval identical to H2A2, together with a normal retrograde atrial activation, is indicative of a bypass of the A-V node. A concealed atrio-His bypass tract must be differentiated also by a concealed septal Kent bundle; also in this case we believe that the detection of an H-Ae interval identical to H2-A2 indicates the former type of bypass. We conclude that a concealed atrio-His bypass can be diagnosed only if, besides the electrophysiologic criteria proposed by other authors, there is an H-Ae interval the same as H2A2.