A hydropic neonate, born at 28 weeks gestation, died 45 minutes after birth. Autopsy showed an abnormally long and tortuous arterial duct which wasoccluded by laminated thrombus. The right heart chambers and the left ventricle were dilated, but there was no other structural cardiac abnormality. Apart from the abnormal ductal morphology, there was no evident cause for the intrauterine thrombosis of the arterial duct, which appeared to be the sole cause of fatal heart failure.