Mitral valve disease is the most common disease of the acquired heart diseases, and atrial septal defect is also one of common congenital heart diseases. Coexistence of these two lesions is rare, but of great hemodynamic interest. Among 443 cases with mitral valve disease and 90 cases with atrial septal defect experienced in the Department of Thoracic Surgery, Seoul National University Hospital, there were 6 cases with atrial septal defect complicated by mitral valve disease. 1. Of the 6 patients, four were female and two were male. The age was ranged from 18 to 46. 2. Atrial septal defect was ostium secundum type in all cases, and the mitral valvular lesions were regurgitation in four and stenosis in two. Type II ventricular septal defect was also combined in one of the cases. 3. The atrial septal defect was corrected by, primary closure and the mitral valve was replaced with the prosthetic or bioprosthetic valve in all cases. The combined ventricular septal defect was closed using Teflon felt patch. 4. The operative result was good except in one who was expired of bacterial endocarditis 4 months after hospital discharge.[KTCS 1979;4:339-345]