Umbilical vessel catheterization was performed in 259 infants in a 3½ -year period, primarily because of respiratory distress. There were 16 clinical complications and 12 autopsy complications in this group of infants. The total complication rate was 9.6%, and the major necropsy complication rate was 4.6%. The most severe of the necropsy complications was thrombosis. In two instances this complication was the possible cause of death. In both cases the infants had severe hyaline membrane disease and required artificial ventilation with high oxygen concentrations. The risk associated with umbilical vessel catheterization is significant, and the use of this technique should be carefully assessed.