Thirty seven patients (mean age = 80.2 years with extremes from 66 to 98 years), presenting atrial fibrillation (AF), presumably idiopathic (non known heart disease, completely normal cardiac auscultation, good quality chest X-ray and electrocardiogram, no laboratory test anomaly), and paroxystic (n = 7: 19 p. cent) or permanent (n = 30: 81 p. cent), were given an electrocardiogram TM and bidimensional. In only 9 of them (24.3 p. cent), this examination is completely normal. Three other patients (8.1 p. cent) present an isolated dilatation of the left atrium. The 25 remaining patients present various cardiopericardic anomalies: valvular pathologies (n = 2: 59.4 p. cent): mitral (n = 15: 40.5 p. cent) more often than aortic (n = 7: 18.9 p. cent); myocardiopathies (n = 8: 21.6 p. cent), hypertrophic (n = 2), dilated (n = 4) or hypertrophic and dilated (n = 2); moderate pericardial effusion (n = 1: 2.7 p. cent). The mean left and right ventricular diameters, measured in TM mode, are normal as well as the mean contractility indexes (percentage of shortening of the small axis, stroke volume) and the mean filling index (mitral gradient EF) of the LV. Overall, the transverse diameter of the LA is moderately increased (41.9 +/- 9.7 mm); it is not significantly different from the AF, either paroxystic (41.5 +/- 4.9 mm) or permanent (42 +/- 9.93 mm) and whether it is (40.5 +/- 9.9 mm =) or not (42.5 +/- 8.8 mm) complicated by a systemic embolism, especially cerebral. Therefore, the sonocardiogram demonstrates a latent cardiopathy in two-thirds of the patients over 65 presenting a presumably idiopathic AF.(ABSTRACT TRUNCATED AT 250 WORDS)