Anomalies of the atrio-ventricular valves are frequent in cases of univentricular heart with severe symptomatology during the first year of life and in necropsy material. This suggests that they are important determinants of the natural history of the malformation. Radical "corrective" surgery, in the presence of these anomalies, may be less successful than palliative programs based on sound physiological grounds. Under such circumstances, enlargement or creation of an atrial septal defect may be necessary. A careful assessment of the state of the atrio-ventricular orifices has to be included in the cardiac catheterization protocol of cases of univentricular hearts. From a morphogenetic viewpoint, a single ventricular cavity is not the appropriate substrate for two normal atrio-ventricular valves to develop.