10 children who died suddenly during liver transplantation were found at necropsy to have extensive obstruction of small lung vessels by platelet aggregates. In 7 of these patients pulmonary artery pressure changes before death were consistent with acute obstruction of the pulmonary vascular bed. Platelet aggregates were not strikingly increased in blood vessels in other tissues. No single obvious cause for these unusual histological findings could be identified, although the presence of intravascular catheters, perioperative blood and platelet concentrate transfusions, and cellular debris from the liver forced into the circulation during surgery might predispose to platelet aggregation.