Five patients with congenital coronary arteriovenous fistula communicating with the right ventricle are presented. Case 5, a 3-week-old infant, represents the youngest patient to be diagnosed and studied, in the literature. The clinical features, hemodynamics and angiocardiographic findings are analyzed. Two major types of fistula affecting the coronary arterial system are recognized. In coronary arteriovenous fistula the functional disturbance consists of a left-to-right shunt, and a diastolic overloading of the left heart chambers and of the right heart chambers into which the fistula opens. In coronary arterio-systemic fistula the altered hemodynamics are essentially those of an internal fistula confined to the systemic circulation. A postulate regarding the nature of the blood flow through the various types of fistulae is given, and this is correlated with the phonocardiographic and hemodynamic findings. The diagnosis should be strongly suspected on the basis of a loud, superficial and continuous cardiac murmur localized in an area atypical for a patent ductus arteriosus. If the continuous murmur is maximal at the pulmonary area, it is extremely difficult to differentiate this anomaly from a patent ductus arteriosus and/or aortic-pulmonary septal defect. Phonocardiographic demonstration of a louder diastolic than systolic component is very suggestive of a coronary arteriovenous fistula communicating with the right ventricle. The roentgenologic and electrocardiographic findings are not specific. Cardiac catheterization is not diagnostic, although when correlated with the clinical picture it may strongly suggest the diagnosis. Angiocardiography, particularly retrograde aortography, is diagnostic. The information obtained from the various methods used is discussed. Dilatation of the ascending aorta demonstrated by angiocardiography was a constant finding in our series. Its pathogenesis is discussed, and its importance as a differential sign in the angiocardiogram against the diagnosis of ventricular septal defect is stressed. The differential diagnosis by retrograde aortography between coronary arteriovenous fistula into the right ventricle and ruptured aortic sinus of Valsalva into the same chamber is discussed and illustrated. While this anomaly is compatible with longevity, the potential complications to which the patient with it may be subjected and the relative innocuousness of corrective surgery without the need for hypothermia or cardiopulmonary bypass, demonstrated in previous reports and in our series, make us believe that surgery should be considered in practically all cases of fistula of the coronary arterial system.