We studied 11 autopsied cases of the Eisenmenger malformation, comparing the findings with 11 hearts with intact ventricular septal structures, and nine hearts having perimembranous ventricular septal defects in the absence of aortic overriding. We found variable lengths for the subpulmonary infundibulum in the hearts with Eisenmenger defects. It was well developed in three hearts, of intermediate length in seven, and very short in one of the specimens. The muscular outlet septum was also of variable length compared with the free-standing subpulmonary infundibular sleeve. Except for one, all hearts had fibrous continuity between the aortic and tricuspid valvar leaflets, such that the ventricular septal defect was then perimembranous. In the remaining case, there was a completely subaortic muscular infundibulum, with the ventricular septal defect showing only muscular borders. The medial papillary muscle was absent in the majority of cases, but was well formed in three hearts, all with relatively short muscular outlet septums. We identified mild, intermediate, and severe degrees of rightward rotation of the aortic valve, and these findings correlated with the extent of aortic valvar overriding. In nine of the 11 hearts, the ventriculo-arterial connections were concordant, but there was double-outlet from the right ventricle in the other two specimens. Based on our anatomic and morphometric observations, we conclude that the hearts we have defined as having Eisenmenger defects show marked individual variation in their specific phenotypic anatomy.