There are few reports that discussed surgical indications and selection of surgical procedures for secundum atrial septal defect (ASD) associated with mitral valve disease from the viewpoint of left ventricular geometry and function. Our study on 20 patients (6 patients with ASD and MS, 14 patients with ASD and MR) indicated that adult patients with ASD and MS can be treated surgically even when left ventricular end-diastolic volume index (LVEDVI) is as low as 35 ml/m2 (one third of the normal level). Left ventricular wall movement returned to normal one year following surgery. Therefore, rather than myocardial impairment, right ventricular volume load disturbing left ventricular dilatation would be the more likely cause of diminished left ventricular function before surgery in the ASD and MS group. In both groups, two patients who initially had undergone mitral valve replacement died from low output syndrome because of the mismatch between the left ventricular volume and the prosthesis used, whereas there were no deaths in the patients who had mitral valve repair. For this reason, mitral valve repair is the treatment of choice for patients with both secundum ASD and mitral valve disease. When valve replacement is the only alternative, selection of prosthetic valves and maintenance of cardiac output by temporary cardiac pacing are important considerations. A low profile valve with a larger orifice area should be selected to avoid the mismatch between a left ventricular volume and a prosthesis.