Hepatic veno-occlusive disease (VOD) is a major regimen-related toxicity after bone marrow transplantation (BMT). Endothelial injury, leading to deposition of coagulation factors within the terminal hepatic venules, is believed to be the key event in the pathogenesis of VOD. To evaluate the benefit and the safety of a VOD prophylaxis with anticoagulants, we conducted a prospective randomized trial of continuous infusion of low-dose heparin among 161 patients under-going either allogeneic (n = 79) or autologous BMT (n = 81). Patients were randomized to receive (n = 81) or not receive (n = 80) prophylactic heparin 100 U/kg/d by continuous infusion from day -8 until day +30 post-BMT. Heparin was found to be highly effective in preventing VOD, which occurred in 11 of 80 patients (13.7%) in the control group versus 2 of 81 (2.5%) in the heparin group (P less than .01). Furthermore, none of the 39 patients in the heparin group developed VOD after allogeneic BMT, versus 7 of 38 (18.4%) in the control group (P less than .01). This prophylactic effect was achieved without added risk of bleeding. Indeed, the low-dose heparin we used did not prolong the partial thromboplastin time and did not increase the red blood cell and platelet requirements. It is therefore recommended that heparin prophylaxis be part of early mortality prevention programs after BMT.