Catheterization of the umbilical artery for the treatment of critically ill neonates provides a convenient method for monitoring blood gas tension and chemistry. The most important complications are thrombotic. Thirty eight aortographs were carried out in infants who underwent umbilical artery catheterization. 17/38 of the aortographs were pathological. Bacterial cultures were positive in 11/17, but only 4 coincided with pathological aortographs. Clinical signs indicating complications due to the presence of the catheter wwere observed in 10 cases. Post-mortem examination of eight subjects--three of whom had pathological aortographs--during the course of the investigation revealed only one case of thrombosis. This baby was considered to have died as a direct result of a thrombotic complication. In our experience the clinical signs of vascular complications and evaluation of the peripheral circulation in the ipsilateral leg remain the most important ways of assessing the indication for catheter-withdrawal.