Fifty seven cases of single (common) ventricle with transposition of the great vessels are reviewed. The diagnosis was confirmed by necropsy in 25 and by angiocardiography in 32 cases. Single (common) ventricle is defined as that condition in which both atrioventricular valves separately enter a single ventricular cavity. According to this definition, cases presenting with common atrioventricular valve, atresia of one atrioventricular valve, or straddling valves were excluded. Two structural types of common ventricle are identified: the left ventricular type and the primitive type of single ventricle. The former showed anatomic features similar to a left ventricle, whereas in the latter, features characteristic of a left ventricle were not present. The latter was considered a more primitive condition than the former. A classification based upon the type of single ventricle and its outflow tract and upon the relationship of the great vessels is presented. Of the 25 cases of single ventricle with transposition studied pathologically, 21 were of the left ventricular type and 4 of the primitive type. Of the 4 cases with double conus 3 were among the 4 examples of primitive ventricle. Among the 21 cases with a single conus, the type of transposition was about equally divided between the d type (10 cases) and the l type (11 cases). Pulmonary stenosis or atresia was observed in 7 of the 21 cases with a single conus and in one of the 4 cases with double conus. In the clinical cases, only angiocardiography could establish the diagnosis and delineate the different types of great vessel ventricular relationships.