We report a case of aneurysm of the atrial septum occurring as an isolated abnormality in a patient with atypical chest pain. Apical two-dimensional echocardiography demonstrated phasic bulging of the fossa ovalis region of the atrial septum into the right atrium. Biplane cineangiography confirmed the presence of a large septal aneurysm in an otherwise normal heart. The incidence, pathogenesis, and complications of this unusual anomaly are briefly discussed.