Radiographic examination represents the only method for the exact localization of intravascular catheters. There has been, however, no review of the technic of umbilical arterial and venous catheterization in newborn infants in the radiographic literature. Moreover, in the evaluation of films of infants with the respiratory distress syndrome, cardiac disorders, or abdominal abnormalities, the radiologist tends to concentrate on anatomical and physiological pathology and pays little attention to the localization or significance of catheters positioned below the diaphragm. Indeed, he may fail to consider the potential complications of the use of such catheters. Catheterization of the umbilical vein was utilized by Diamond et al. (1) in 1946 for the purpose of exchange transfusion in infants with erythroblastosis fetalis. Later, catheterization of the vein, as well as of the umbilical artery, was used for angiocardiography (3) and now is employed quite routinely for withdrawal of blood for gas analysis and for the introduction of fluids or blood into the vascular system in infants with the respiratory distress syndrome. To understand the positioning of umbilical catheters, knowledge of the anatomy of the neonatal vascular circulation is essential. Figure 1 illustrates this circulation diagrammatically. The course of the catheter into the aorta is relatively simple. It passes caudally in the umbilical artery, angulates near the dome of the bladder to enter the hypogastric artery, and then goes posteriorly and craniad to the common iliac and thence to the abdominal aorta. The course of the venous catheter is more complicated. Following passage through the umbilical vein, the catheter traverses the ductus venosus (which remains patent for the first fifteen to twenty days of life) and then enters the inferior vena cava. It may enter the right atrium or pass through the foramen ovale into the left atrium. The catheter may be diverted into the portal sinus and thence into a portal vein, or it may enter the splenic or inferior mesenteric veins. Introduction of a catheter into the aorta is relatively easy since the only difficulty encountered is the angulation at the confluence of the umbilical and hypogastric arteries. The placement of the venous catheter is much more difficult. When catheterization is performed in cases of respiratory distress syndrome, the optimum location of the catheter tip is in the relatively short segment of inferior vena cava near the diaphragm, proximal to the entrance of the hepatic vein and below the right auricle. Dunn (2) in 1966 correlated the length of catheter from the umbilical vein to this point in the inferior vena cava with the size of the infant and found that the catheter length varied from 5 to 10.5 cm. He felt that the ratio of the catheter length to the shoulder-umbilicus length was most reliable.