We present an eight year-old girl who required an operation for moderate mitral insufficiency associated with partial atrioventricular septal defect. Echocardiography disclosed an ostium primum atrial septal defect and double-orifice mitral valve with moderate mitral regurgitation secondary to a cleft in the anterior leaflet and prolapse of the anterior leaflet. Intraoperative inspection revealed that the chordae from each orifice were attached to a single papillary muscle which resulted in a unique double-orifice mitral valve. Mitral valve repair using chordal shortening and cleft closure was successfully performed. Postoperative echocardiography observed trivial MR and no mitral stenosis.