Due to the accumulation of evidence concerning a putative role of nitric oxide (NO) in graft-versus-host disease (GVHD), we performed follow-up measurements of the stable end-products of NO, nitrite/nitrate (NO 2-/NO 3-) in plasma of patients undergoing allogeneic (n= 16) and autologous (n= 6, as a control) bone marrow transplantation. NO 2-/NO 3-concentrations were set in relation to the clinical course and to serum levels of soluble tumor necrosis factor receptor 75 (sT-NFrec 75) and neopterin, both of which are known to be sensitive indicators of cellular immune activation phenomena involving macrophages in vivo, and endogenous interleukin (IL)-10, a major T helper cell type 2 (TH-2)-derived cytokine and potent inhibitor of macrophage activation and NO formation.