Non-rheumatic mitral regurgitation (MR) was found in 33 patients (10.2%) in a total of 324 patients with secundum atrial septal defect (ASD) who were operated on during 13 years period at the Mitsui Memorial Hospital. This combination was more frequent in women (27 patients: 13.5%) than in men (10 patients: 6.5%), and the incidence tended to increase with age. Inspection at operation revealed a marked fibrous thickening (ASD plaque) of the anterior and posterior mitral valve leaflets with a localized bulging into the left atrium at the posteromedial commissural area. Similar but less prominent change was found in nearly all patients above 20 years of age in the absence of mitral regurgitation. A friction between the anterior and posterior mitral leaflets caused by the abnormal left ventricular motion secondary to the right ventricular volume overload might be responsible to the genesis of this lesion. I proposed 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 mitral or tricuspid involvements. As ASD plaque begins to appear in the second decade, surgical closure of the defect should be indicated before ten years of age. The incidence of severe MR or tricuspid regurgitation (TR) was significantly higher in patients older than 40 years of age. In cases with ASD-MR complex with or without TR, mitral or tricuspid annuloplasty with direct closure of the defect should be recommended.