Ebstein's anomaly of the tricuspid valve is associated with right ventricular dilatation in approximately two-thirds of cases. Dilatation may be massive. It involves not only the right ventricular wall proximal to the tricuspid valve (atrialized ventricle) but also the right ventricle distal to the valve (funtional right ventricle), including the right ventricular infundibulum. For further definition of the pathogenesis of dilatation of the functional right ventricle (distal chamber), a morphometric histopathologic study was performed on 10 hearts with Ebstein's anomaly and 10 normal age-matched control hearts. In the group with Ebstein's anomaly, five hearts exhibited dilated ventricles and five did not. The study demonstrated that dilatation of the right ventricle in Ebstein's anomaly was associated not only with thinning of the wall but also with an absolute decrease in the number of myocardial fibers counted through the thickness of the wall from endocardium to epidcardium.