Twenty-three infants less than 1 year of age with symptoms due to patent ductus arteriosus have been studied clinically and by cardiac catheterization. In view of the absence of a characteristic clinical picture in this age group, it is suggested that the diagnosis be considered in any infant with clinical evidence of a large left-to-right shunt. Cardiac catheterization has proved to be a useful and reliable method for establishing the diagnosis and providing detailed hemodynamic observations. The problem of arterial oxygen unsaturation, present in over one-half the patients, is discussed in its relation to pulmonary disease and right-to-left shunting of blood. A large atrial left-to-right shunt may occur through a dilated foramen ovale in infants with patent ductus arteriosus. The significance of this is considered. Pulmonary arterial hypertension of mild to severe degree was present in the majority of patients but pulmonary vascular resistance was elevated in only five infants. The absence of a continuous murmur in infants with patent ductus arteriosus is discussed in relation to time pressure gradient between the aorta and pulmonary artery. Surgical treatment of infants with severe symptoms due to patent ductus arteriosus has produced gratifying results.