67 patients presented with a syncope of unknown origin following the usual exploration and were the subject of a more elaborated study: echocardiography (echo 2D) and Holter. 3.5 per cent of the Holter recordings enabled to ascribe to the heart the cause of the syncope. More than half of the Holters disclosed abnormalities of the cardiac rhythm leading to an anti-arrhythmic therapy in some cases, even when the cardiac origin of the loss of consciousness (LC) could not be confirmed. Echocardiograms could not lead to the diagnosis and less than one fourth of them disclosed abnormalities. In a developing study (14.8 +/- 8.7 months) that was carried out, 3 out of 4 patients did not present with any more LC; 3 died (6%) and 9 (18%) had LC again. On the other hand, in 5 patients (10%) a diagnosis could be evoked at the end of this follow-up. This study confirms the difficulties of diagnosis of this group of syncope of unknown etiology, as evidenced by the low results of intense and costly explorations and the little information obtained, even after a follow-up of more than 1 year.