A 25 year old woman with effort intolerance and systemic venous congestion since childhood, and restrictive ventricular filling and severe atrioventricular (A-V) valve regurgitation at cardiac catheterization died after valve replacement surgery. At autopsy, the atria were enlarged and thickened; both ventricles were small. In some areas the right ventricular wall was only a few myofibers thick. The myofibers of both ventricles were hypertrophied, and there was widespread fiber disarray in the left ventricular free wall, the interventricular septum and, focally, the right ventricular free wall. Because the ventricles were not enlarged or thickened despite myocardial fiber hypertrophy and severe chronic A-V valve regurgitation, and the myofiber disarray was widespread, this is a very unusual case of biventricular hypoplasia and dysplasia.