Twenty consecutive patients with ventricular septal defect and aortic valve prolapse were evaluated by cross-sectional echocardiography. Angiographic confirmation was available in all and surgical confirmation was found in 17. In 19, the right coronary cusp was involved and appeared to plug the defect in the precordial long- and short-axis cut. The cusp was deformed and appeared to pivot from the crest of the interventricular septum. In all 19 angiography demonstrated prolapse of the right cusp. Noncoronary cusp prolapse was observed in two by cross-sectional echocardiography and in six by angiocardiography. The ventricular septal defect was perimembranous in 14 and doubly committed subarterial in six by echocardiography. Angiographically, the ventricular septal defect was felt to be perimembranous in 15 and doubly committed in five. Aortic regurgitation was detected by Doppler interrogation in seven, all of whom underwent plication of the right coronary cusp. Angiographic evidence of regurgitation was noted in 11, but four were mild and possibly related to catheter position. Five patients had associated muscular right ventricular outflow tract obstruction and three had a subaortic ridge. Combined cross-sectional and pulsed Doppler echocardiography provide a reliable assessment of right coronary cusp prolapse associated with a ventricular septal defect. Noncoronary cusp prolapse appears more difficult to detect. This technique should help optimize the management of patients by providing a means of early detection prior to the development of aortic regurgitation.