Non-rheumatic mitral regurgitation was found in 9.1% (27 patients) of all patients with secundum atrial septal defect (298 patients) who were operated on during a 13-year period at the Mitsui Memorial Hospital. This combined lesion was more frequent in female (12.3%, 19 patients) than male (5.6%, 8 patients), and the incidence tended to increase with age. Inspection at operation revealed a marked fibrous thickening of the anterior and posterior mitral valve leaflets with a localized bulging into the left atrium in the posteromedial commissural area. Similar, but less prominent change was found in nearly all patients above 20 years of age with mitral regurgitation, although the small area where the commissural chordae were attached was not involved. A friction between anterior and posterior mitral leaflets due to abnormal left ventricular motion secondary to the right ventricular volume overload might be responsible to the genesis of this lesion. Mitral annuloplasty was performed on 21 of 27 patients with a satisfactory result without operative mortality. Except one case of early death from cerebral thromboembolism they were all restored to a functional status of class I by NYHA standard within one year after operation, and there were no late death during a follow-up period of as long as 12 years. We propose a new entity "ASD, MR complex" for this combined lesion, as it demonstrates a specific pathophysiology, and necessitates a different procedure for surgical correction from that of a secundum atrial septal defect without other anomalies.