A 20 days old female baby presenting with dyspnea was found to have coarctation of the aorta and patent ductus arteriosus by echocardiography. The Doppler spectral display disclosed mainly a left-to-right shunt through the ductus. Therefore, the prostaglandin El infusion was discontinued. She developed acute left heart failure, and the echocardiograms at that time showed a markedly narrowed juxtaductal aortic coarctation with a closed ductus. After resumption of prostaglandin El, with an attenuated juxtaductal aortic coarctation, the left ventricular contractility and the systemic perfusion improved. Therefore, we conclude that the central role of a patent ductus in such a patient is to avoid the ductal constriction which will worsen the aortic coarctation.