Mesenteric blood vessels were examined in postmortem specimens of 30 selected infants. The infants had had necrotizing enterocolitis (16 cases) or other lesions (14 cases). Mesenteric thromboemboli were observed in 12 of the infants with necrotizing enterocolitis (P less than 0.001). Umbilical vessel catheters were apparently responsible for the emboli. Multiple instances of thrombosis and thromboembolism were detected in other organs. Ninety-four percent of infants with necrotizing enterocolitis were orally fed (P less than 0.001). Mesenteric thromboemboli were more frequent in infants who were fed via the digestive route while an umbilical arterial catheter was in place. Mesenteric thromboembolism in these infants was postulated to have been attributable to increased postprandial blood flow in the mesenteric vessels.