We evaluated the efficacies of serum catalase (CAT), 5'-nucleotidase (5'NT), and tumor necrosis factor-alpha (TNF) as diagnostic markers of acute graft-vs-host disease (GVHD) in 28 allogeneic bone marrow transplant recipients by comparing their abilities to discriminate between GVHD-related and non-GVHD-related complications. Mean peak serum CAT concentrations for patients with GVHD-related complications (n = 17) were about fivefold higher than concentrations in patients with non-GVHD-related complications (n = 25; P = 0.003), whereas the mean peak concentrations of serum 5'NT and TNF were not substantially different. Similarly, the sensitivity and specificity of serum CAT (100% and 88%, respectively) for use as a diagnostic marker of GVHD were much better than those of serum 5'NT (88% and 24%, respectively) or serum TNF (65% and 4%, respectively). Receiver-operating characteristic plots of all possible sensitivity-specificity pairs obtained over the whole range of results also showed that serum CAT has the best diagnostic accuracy. Low specificities of serum TNF and 5'NT were caused mainly by their increase in septicemia, fungal infection, and veno-occlusive disease and after the use of granulocyte-macrophage colony-stimulating factor to stimulate donor cell engraftment. Serum CAT may prove to be a rapid and relatively noninvasive test for the diagnosis of acute GVHD.