In an autopsy material of 29 cases of the hypoplastic left heart syndrome coronary fibroelastosis was found in 1 case, endocardial fibroelastosis in 8 cases. Figures for 10 cases of the hypoplastic right heart syndrome were 6 cases of coronary fibroelastosis and 1 case of endocardial fibroelastosis. Age ranged from stillborn up to 11-1/2 months. Coronary and endocardial fibroelastosis seemed to be mutually exclusive localizations of congenital fibroelastosis since in our material they did not occur together in the same hearts. In hypoplastic right hearts coronary fibroelastosis was either restricted to the right coronary artery (right circumflex and posterior interventricular branch), or it was found also in the left coronary artery (anterior interventricular branch), with the most serve affections always being situated in the right one. In the only case of coronary fibroelastosis among the hypoplastic left hearts the condition was limited to the anterior interventricular branch of the left coronary artery which communicated with the hypoplastic left ventricle by a fistula. Coronary fibroelastosis was exclusively found in branches supplying the hypoplastic right ventricle and/or in a branch connected by a fistula to the hypoplastic left or right ventricle. Endocardial fibroelastosis was generally found in hypoplastic left ventricles with either no outflow or with severe outflow obstruction. A theory concerning the aetiology of both coronary and endocardial fibroelastosis of the hypoplastic ventricles is proposed. It is argued that development of fibroelastosis may in both localizations be caused or favoured by the coincidence of two factors: abnormal haemodynamic conditions and poor oxygenation of blood and tissues. Observations made in a reference material of 35 hypoplastic left and 24 hypoplastic right hearts were in accordance with this view.