SummaryNcopterin, a precursor of biopterin, is released by macrophages during allogeneic response and in vitro data suggest that the release ofγ-interferon by the T-cell triggers neopterin excretion. In order to determine the clinical relevance of serum neopterin levels, serial serum samples of seven allogeneic bone marrow recipients were measured by a radioimmunoassay and evaluated retrospectively. As reported by others, very high levels of neopterin:±100 nMol/L (n:<10) are observed in patients with graft-versus host-disease (GVHD) or when severe viral infection occured and in all cases, the rise of neopterin level preceeded and/or correlated with the clinical manifestations. The reappearance of leukemic blasts was also associated with rising neopterin level, but the value never reached 50 nMol/L. Although neopterin as a marker is not specific for any of these complications, its daily evaluation allows to identify patients at risk for major clinical complications. Moreover, the striking increase of neopterin level during GVHD suggests thatγ-interferon plays a role in the pathogenesis of this reaction. Further investigations are needed to better document this hypothesis.