Idiopathic bilateral atrial dilatation is extremely rare. We are reporting a case in a 79 year-old patient, presenting a picture of total cardiac insufficiency. The positive diagnosis was established by bi-dimensional sonography and right angiography. Nuclear magnetic resonance confirmed the diagnosis and specified the size of the various cavities. From a rhythm standpoint, there was an atrial fibrillation without conduction disorders. The main factor of the cardiac insufficiency seems to be a low atrio-ventricular output, since the valvular insufficiency due to annular dilatation is only a secondary factor. The etiology is unknown, but a congenital origin seems most probable without excluding the possibility of an acquired structural disorder.