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: There was no septal malalignment with a normal aortic valve position. 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. 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. The majority of cases showed a normally formed aortic valve but with poor support. 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. 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: There was mild to moderate dextroposition (or levoposition) of the aortic valve due to a septal malalignment.(ABSTRACT TRUNCATED AT 400 WORDS)