Angiographic criteria for the recognition of aortic valve prolapse in isolated ventricular septal defect were based on the degree of aortic cusp deformity and the presence or absence of aortic regurgitation. Ninety eight consecutive patients with isolated perimembranous or infundibular ventricular septal defects who were catheterised and had aortography performed were reviewed. They included five with postoperative ventricular septal defects and three with additional mild right ventricular outflow tract obstruction. Eighteen were found to have aortic valve prolapse. Although eight of the 18 were noted to have aortic regurgitation angiographically, only three had an early diastolic murmur. Only eight of the 18 patients had cross sectional echocardiographic findings suggestive of prolapse. All of these had at least moderate prolapse angiographically. Cross sectional echocardiography was found to be insensitive in diagnosing mild degrees of aortic valve prolapse. A trend towards a decreasing left to right shunt was noted as the degree of aortic valve prolapse increased. Spontaneous decrease in the size of a ventricular septal defect may be due to unrecognised aortic valve prolapse without clinical evidence of aortic regurgitation.