We prospectively studied the echocardiographic findings of an atrial septal aneurysm (ASA) to estimate its incidence and to clarify its clinical characteristics and significance. Post-mortem examination was also performed in three patients. Echocardiographically, ASA was defined as a bulging segment of the atrial septum localized in the fossa ovalis, either fixed in one direction or oscillating between the atria. It was classified in three types according to Hanley et al., i.e., Type 1A, protruding into the right atrium without oscillation; Type 1B protruding into the right atrium with oscillation, and Type 2, protruding into the left atrium with oscillation. Among 2,074 consecutive subjects in the echocardiographic study population, ASA was diagnosed in 26 patients (1.2%). This figure was slightly higher than those previously reported (0.6% to 1.0%). The extent of protrusion of the aneurysm was 8 mm or more in all patients, regardless of its direction, and it was assumed that this is a reasonable echocardiographic diagnostic criterion in the apical four-chamber view. All patients were over 51 years in age, with a mean of 71 years. Most patients (96%) had oscillation of their aneurysms. Twenty-one patients (81%) were of Type 2; one was Type 1A, and four were Type 1B. Post-mortem examination of three patients revealed septal protrusion toward the right atrium in all, and patent foramina ovale in two of them. Among the 26 patients, two (8%) had systemic embolic complications; one, cerebellar infarction, the other, cerebral infarction and mesenteric artery embolism. In conclusion, atrial septal aneurysm was observed in 1.2% of subjects undergoing routine echocardiography, with a distinctive distribution among patients over 51 years of age. Characteristically, it protrudes into the left atrium 8 mm or more, and it is sometimes associated with patent foramen ovale. Systemic embolism is a possible complication of this anomaly.