It has been reported that serum levels of thrombomodulin (TM) reflect endothelial damages in various diseases. We measured serum TM levels between day-10 and day 100 in 6 patients receiving allogeneic bone marrow transplantation. Serum TM levels were increased when patients had transplant related complications including graft versus host disease, hemorrhagic cystitis and interstitial pneumonitis. In patient without complications, serum TM levels were within normal limits. These results suggest that the serum TM level serves as a useful marker of treatment related toxicity and a predictor of complications after BMT.