Paroxysmal supraventricular tachycardia (SVT) was studied in 46 patients with normal electrocardiogram (ECG) during sinus rhythm (SR), All of them had had at least one SVT crisis. They all complained of palpitations starting and ending abruptly, and of more than one hour duration. There were never seen ECG alterations suggesting ventricular preexcitation during SR. An electrophysiologic study using atrial and ventricular programmed stimulation and intracardiac electrograms, was used to establish the diagnosis. There was no significant difference in sex or age distribution according to the ethiologic diagnosis. It was found that 67.4% had a concealed accessory bundle, responsible of the SVT, 32.6% had A-V nodal reentry (p less than 0.01). Of all concealed accessory bundles, 67.7% were left lateral Kent (45.6% of all patients). It is discussed the possibility of antegrade conduction through concealed bundles during SR and that this may be seen by means of a M-mode echocardiography. We conclude that neither sex nor age help to suspect ethiologic diagnosis; the most common accessory bundle is the left lateral Kent and that in some cases it can be detected by M-mode echocardiography. This accessory bundle is the most usual cause of SVT in patients with normal ECG during SR.