1. A prospective study to determine the frequency of clinically occult arterial thromboembolism in neonates harboring an umbilical artery catheter was conducted, utilizing 100 consecutive infants admitted to our neonatal intensive care unit. Autopsies were also performed in 12 of the 16 infants who expired during the study. 2. Pull-out aortography was randomly done in 21 infants, or approximately one-third of those in whom an arterial catheter had been inserted. Significant thrombus formation was demonstrated in 18 of the 19 infants with a technically satisfactory aortogram. One infant died as a direct result of the intravascular thrombosis. 3. Thromboembolism may occur in new-born infants and produces little or no clinical evidence of its presence. Some of the ischemic signs and symptoms encountered in neonates harboring umbilical artery catheters and attributed to vascular spasm are probably manifestations of thromboembolism. 4. The potential hazard of clot formation following umbilical artery catheterization should prompt efforts to minimize its occurrence, including careful selection of patients, development of nonthrombogenic catheters and the employment of improved anticoagulation techniques. 5. The high incidence of occult thrombosis demonstrated in this study has prompted us to position the umbilical artery catheter at the aortic bifurcation to minimize the extent of thrombus formation and the likelihood of thromboembolism to vital visceral vessels. 6. Although no immediate complications were noted in those surviving infants with roentgenologically demonstrable clots in this study, close follow-up observation is warranted to exclude the possibility of long term sequelae.