The technique of umbilical artery catheterization is described. Complications occurred in 10% of a total of 335 insertions in the years 1963-66. 13 babies had haemorrhages none of which were serious. 17 showed clinical signs ofarterial obstruction; in 16, these signs disappeared on removal of the catheter; 1 had arterial thrombosis at necropsy. In 4 other fatal cases arterial thrombosis was not detected before death. Only 1 survivor had any sequelae, and lost the tip of a toe. Hypertonic solutions should not be injected through the catheters. Umbilical artery catheterization is a simple and sometimes a necessary procedure, but not one without danger.