Coagulation studies were performed on 13 children with clinical diagnoses of Reye syndrome. Four had abnormalities so severe that transfusion of coagulation factors and platelets were required prior to percutaneous liver biopsy. In one of these, abnormalities frequently found in association with acute hepatic failure were present. Disseminated intravascular coagulation was noted in two, while in the fourth child laboratory data were most consistent with that process. Although the coagulation defect associated with Reye syndrome is most often due to decreased production of all coagulation factors except VIII, these patients demonstrate that consumption can also occur. The severity of the coagulopathies found indicates that bleeding can become a major complication in the diagnosis and management of Reye syndrome.