Fifty seven patients underwent repair of a partial atrioventricular septal defect from January 1980 to December 1986. The ostium primum atrial septal defect was closed with autologous or bovine pericardium. The cleft in the anterior mitral leaflet was present in 53 cases, absent in 4 cases. Of the 53 cases with a cleft in the anterior mitral leaflet, 48 received suture repair of the cleft, 3 received mitral valve replacement. There was no hospital death and all the patients were followed-up for a mean period of 26.4 months. Four required permanent pacemaker implantation due to complete heart block, and one of them died of sudden malfunction of pacemaker. Two received reoperation due to significant residual mitral insufficiency. Suture repair of the cleft in the anterior mitral leaflet resulted in significant decrease in degree of mitral regurgitation. During follow-up period 49 patients were in NYHA class I, 7 patients were in NYHA class II. This report suggests that excellent result can be achieved from repair of the partial atrioventricular septal defect by managing the left A-V valve as a bileaflet structure.