Veno occlusive disease (VOD) is a frequent complication of allogenic bone marrow transplantation (BMT) for which no predictive blood markers are available. 39 patients grafted for severe aplastic anemia (18), and leukemia (21) were prospectively studied. Of the 39 patients, 5 leukemic patients, but no aplastic patients developed VOD. In all the 5 patients with VOD complications we demonstrated a decrease in factor VII and in protein C before the clinical onset of the disease and before any changes in hepatic enzymes were observed. This decrease is the earliest sign of hepatic involvement by the VOD suggesting that the determination of Factor VII and protein C can be used as a prediction test to identify the patients who are at risk of developing VOD after transplantation. In addition, a toxicity of the endothelial cells was suggested by the observed increase in von Willebrand factor and in Serum Angiotensin Converting Enzyme. Signs of endothelial toxicity was more pronounced in leukemic than in aplastic patients.