In a 53-year-old woman with cardiomegaly and atrial fibrillation, cineangiography revealed idiopathic enlargement of the right atrium and mitral valve prolapse. Two-dimensional echocardiography was carried out in the entire family, which lives in the northeastern part of Switzerland. In 2 of the 4 siblings of the proposita, idiopathic enlargement of the right atrium, mitral valve prolapse and supraventricular rhythm disturbances were found. In one child of the patient with the largest right atrium, idiopathic enlargement of the right atrium associated with mitral valve prolapse was also present. To our knowledge this is the first report on familial occurrence of idiopathic enlargement of the right atrium accompanied by mitral valve prolapse.