Our histological study of the ductus arteriosus compared 12 infants who were treated with E-type prostaglandins with 12 control infants matched as closely as possible for age and diagnosis. Distinguishing histological features were seen in five of the treated cases and in none of the untreated cases. These features were intimal tears in two and hemorrhage into the media in five. Medial edema, mural thrombosis, and interruption of the internal elastic lamina at sites not beneath intimal cushions occurred both in treated and untreated infants and therefore were not distinguishing features. The gross anatomical features of the ductus arteriosus were assessed in these 24 and in another 85 cases. Significant differences in the ductus arteriosus were noted between cases with pulmonary atresia or aortic atresia and normals, but not between those treated with E-type prostaglandins and controls.