The authors measured serum neopterin by HPLC in 3 patients with hemophagocytic histiocytosis (HH) with DIC which occurred during the clinical course of T cell malignant lymphoma (T-ML). Extremely high levels of serum neopterin (754.3 +/- SD 467.3 pmol/ml) were found in the patients, about 200, 13 and 10 times higher than that of normal subjects, non-HH DIC patients and non-HH/DIC T-ML patients. Moreover their serum soluble interleukin-2 receptor levels were markedly elevated. These results indicate that markedly raised serum neopterin would be a sensitive parameter for activation of the mononuclear phagocyte system closely associated with T-cell activation and may serve as a useful diagnostic marker for HH.