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Persistent common atrioventricular canal is one of the forms of congenital heart disease which stems from the failure of fusion of the endocardial cushions to each other in the embryonic stage. This deficiency in its complete form shows a common cleft in the anterior mitral and septal tricuspid valve leaflets and defects of the intracardiac septum above and below, allowing communication among all four chambers of the heart. The superior edge of the ventricular septum stands free. In the least deficient form, referred to as partial , fusion of the endocardial cushions is incomplete on the left side only, resulting in a normal septal tricuspid leaflet and a cleft anterior mitral leaflet. There is a pattern which may be called intermediate between the complete and partial forms, in which the only evidence of fusion of the endocardial cushions is a small bridge of valvular tissue seen on the superior aspect of the ventricular septum. Common to all three types of persistent common atrioventricular canal is persistence of the ostium primum, which probably is due, at least in part, to failure in the upward growth of the endocardial cushions. The pathologic data compiled are based on 28 cases of persistent common atrioventricular canal, of which 16 represented the complete, 7 the intermediate, and 5 the partial type. Subvalvular spaces allowing free interventricular communication were present in 13 of the 15 hearts with the complete form, in 4 of the 7 with the intermediate form, and in 1 of the 5 with the partial form. The lower limbus of the fossa ovalis was deficient in 13 of the 28 specimens. Nine of these defects were in hearts with the complete form of the malformation, 3 were associated with the intermediate form, and 1 with the partial form. Cardiac enlargement, including right ventricular hypertrophy, was common. In several hearts biventricular hypertrophy was evident. Not uncommonly, one or more associated cardiac defects were seen in the speciment studied. Atrial septal defect was noted in 12 specimens, double mitral orifice in 5, and miscellaneous types of defects, including a case of significant patent ductus arteriosus, in 7 others. Among a total of 109 cases of persistent common atrioventricular canal examined or documented for tabulation are 24 instances with additional cardiac or great-vessel malformations other than patent foramen ovale and patent ductus arteriosus. Six examples of subacute bacterial endocarditis complicating this lesion have been documented thus far. The rare anomaly of double mitral valve was observed in 5 of the 28 hearts with persistent common atrioventricular canal. The frequent association of these two lesions has not been commented on before.