Prostaglandin E1 was utilized for maintaining the patency of the ductus arteriosus in 19 infants with congenital lesions whose survival depended on ist patency. Fourteen of the patients had lesions which required blood flow from the aorta to the pulmonary artery (cyanotic group); 11 of these were considered successful. Two of the failures were in older infants aged 13 days and 1 month. The remaining five trials were in patients who required a flow of blood from pulmonary artery to aorta (left heart and aortic defect group), two of whom were considered to have responded successfully to the prostaglandin E1 administration. Based on this experience, it is suggested that this method of treatment is of benefit in the cyanotic group in providing emergency medical care while awaiting surgical palliation. A similar benefit in the aortic arch defect group would seem to be less well demonstrated at this time.