Sixty patients (pts) with very disabling symptoms during supraventricular tachycardia (SVT) underwent electrophysiological study to determinate the mechanism of arrhythmia. 20 pts had, during normal sinus rythm (NSR), ECG pattern of ventricular pre-excitation (V-P); 40 pts, instead, had ECG pattern within normal limits (WNL). In V-P pts, circus movement tachycardia by overt accessory pathway (AP) was initiated. Among ECG-WNL pts group: 10 pts showed ectopic atrial rythm; 22 pts had SVT due to A-V nodal reciprocation; and 8 pts revealed the participation of concealed AP in the reentrant circuit of tachycardia. Among these latter, atrial stimulation at various sites and drugs-administration discovered extranodal by-pass liable for concealed V-P in 2 pts; in the remaining 4 pts it was possible to demonstrate concealed atrial preexcitation by ventricular stimulation, during NSR and SVT, and by atrial mapping during ventricular stimulation and SVT. Our report confirms the significative incidence of concealed AP at the basis of numerous cases of SVT (26%) and outlines the distinction between the latent W.P.W. syndrome and the atrial preexcitation, due to anomalous by-pass with anterograde block.