A 36-year-old woman was admitted to our hospital with exertional dyspnea, palpitation and cardiac enlargement on standard chest X-ray. Physical examination did not reveal any significant abnormality and routine laboratory tests were normal. ECG showed atrial fibrillation and negative T waves in lead III and aVF, with a stiffened ST segment. M-mode echocardiography showed absence of pericardial effusion and normal size of both ventricular chambers. Two-dimensional echocardiography showed a large saccular dilation of the right atrium without significant enlargement of other cardiac chambers. A slight displacement of tricuspid valve leaflets was also present. Cardiac catheterization was effective in the diagnosis of idiopathic enlargement of the right atrium. Although two-dimensional echocardiography has some limitations in the diagnosis of this unusual malformation, it may prove useful in the noninvasive follow-up of these patients, in order to monitor the eventual increase of atrial size and to promptly detect late complications.