Making a diagnosis of a patency of a ductus arteriosus is often difficult in the first few months or first year of life. When the diagnosis is in doubt, one may profitably turn to angiocardiography for help. In this paper a method of demonstrating the patent ductus by the injection of contrast medium is described. The practical importance of the correct diagnosis is presented with reference to 4 cases treated surgically in the early months of life. Demonstration of coarctation of the aorta by this method is also discussed.