In an attempt to clarify the pathogenetic morphology of aortic regurgitation (AR) due to prolapse of the aortic valve (prolapsing AR) associated with ventricular septal defect (VSD), 201 specimens from Japanese autopsy series with isolated VSD were examined. Among these hearts, there were 128 cases (64%) of infundibular VSD (IVSD); 29 of them (14%) showed AR due to prolapsed cusp, of which nine cases developed a large aneurysm of the sinus of Valsalva. Another 32 cases (16%) had varying degrees of prolapse but without AR and were considered to show the prodrome of prolapsing AR. These 61 cases (30%) were examined with special reference to the type of septal alignment, location of the defect, relation of the defect to the aortic valve, and anomalies of the aortic valve and sinus of Valsalva. There were two principal forms in this syndrome: The common form, i.e., simple punched-hole IVSD with normal septal alignment in 82% (50/61) of cases, and a rare form, i.e., malalignment IVSD in 18% (11/61) of cases. The latter included Eisenmenger-type IVSD due to anteriorly deviated outlet septum (10/11 cases) and coarctation-type IVSD due to posteriorly deviated septum (1/11). Both forms had several subtypes according to the location of the defect, i.e., subpulmonic, muscular, perimembranous, and total IVSD. The relevant anatomical findings of the common form of the syndrome were: (1) There was no septal malalignment with a normal aortic valve position. (2) The VSD was a simple muscular defect in any part of the infundibular septum between the pulmonary valve above and the membranous septum below, the majority of cases (80%), however, showed subpulmonic IVSD. (3) The annulus and sinus of Valsalva wall of the right coronary cusp, which is normally supported firmly by this septum, became exposed in the muscle defect and were poorly supported. (4) The majority of cases showed a normally formed aortic valve but with poor support. (5) The muscular defect was relatively large, but the functioning VSD was usually less than moderate in size with a half-moon shape below the denuded sinus of Valsalva wall and annulus. (6) The functioning VSD appeared to become narrower depending on the degree of prolapse into the defect, resulting in a crescent-moon or slit-like shape, and it may close in rare cases. The major anatomical findings of the rare form were: (1) There was mild to moderate dextroposition (or levoposition) of the aortic valve due to a septal malalignment. (2) The size of the VSD was moderate or less with a location in any part of the infundibular septum, but most commonly involving the perimembranous infundibular portion (70%). (3) It was always associated with anomalies of the aortic valve, such as bicuspid valve or uneven division of the cusps, and a deformed wall of the sinus of Valsalva, which appeared to be related to a less than moderate septal malalignment. (4) The deformed aortic valve was exposed in the defect. The development of the prolapsing AR in the two forms of the syndrome could be attributed to the interaction between the above-mentioned anatomical defects and hemodynamic factors, among which the most important was the “Venturi effect” caused by a rapid blood flow through the functioning VSD according to Bernoulli's theorem. With progress of the disease, this syndrome is clinically subdivided into three stages: The prolapsing stage which is characterized by the Venturi effect, the reversible AR stage which has AR and the Venturi effect, and the irreversible AR stage in which the deterioration of the aortic valve is complete. Early surgical intervention to stop the Venturi effect is recommended, at least at or preferably before the reversible AR stage, even in those patients with malalignment-type VSD who have inherent anomalies of the aortic valve and its apparatus.