Thirty-two patients with interventricular septal defect (IVSD) with aortic insufficiency are described, twenty of whom have been operated on. The aortic insufficiency was generally slowly progressive in the cases in which this could be determined. A right ventricular subpulmonic valve gradient was found in 45 per cent (thirteen patients), in about half of whom (six patients) this gradient was due to prolapse and herniation of the right aortic cusp through the IVSD. This is a much higher incidence than has been appreciated. Of the twenty patients operated on, none had aortic valve replacement, six had accompanying annuloplasties and fourteen had IVSD closure only. In none was the aortic insufficiency made worse, and in only five was the aortic insufficiency definitely unbenefited. In three cases of just IVSD closure the aortic insufficiency completely disappeared postoperatively, and in at least four others the aortic insufficiency was significantly reduced in severity. It is concluded that patients with this combination of lesions should be considered for closure of the IVSD in order to prevent progressive aortic insufficiency. © 1972.