Thrombotic complications may occur early after marrow transplantation and many data suggest that endothelial injury plays a pivotal role in their pathogenesis. Since plasma thrombomodulin and P-selectin are thought to be of value as markers of vascular endothelial cell membrane injury, we investigated their plasma concentration in bone marrow transplant patients aiming better to clarify the degree of endothelial involvement. Plasma thrombomodulin and P-selectin were monitored in 25 patients without thrombotic complications before transplant, on day 0 and weekly for 1 month thereafter, while in three patients who developed VOD monitoring continued until day +52. These proteins were in the normal range in all the uncomplicated patients and in two with reversible VOD, while they were always very high in the only patient who developed very severe and lethal VOD. In conclusion, we suggest that endothelial activation/damage occurs rarely in the course of BMT for hematological malignancies; we were able to document endothelial injury in only one patient with very severe thrombotic complication.