To determine the pathogenesis of hemophagocytic lymphohistiocytosis (HLH), serum levels of neuron-specific enolase (NSE) and cytokine profiles were investigated. Serum concentrations of NSE and several cytokines were measured by immunoassays, and the association was evaluated in 18 HLH patients. Serum NSE levels increased (> 10 ng/mL) in 27/29 samples (93%) during the active febrile phase, the mean level of which (35.9 ng/mL) was much higher than that during the remission phase (11.2 ng/mL) (P = .001). The peak levels of NSE in 11 patients who required cytotoxic agents were higher than those in 7 patients without chemotherapy, 64.6 +/- 49.4 and 17.9 +/- 12.9, respectively (P = .265). The NSE levels correlated positively with the levels of interferon (IFN)-gamma (Pearson's correlation coefficient [r] = 0.408, P = .044), soluble interleukin-2 receptor (sIL-2R) (r = 0.464, P = .048), lactate dehydrogenase (r = 0.830, P < .00001), aspartate aminotransferase (r = 0.531, P = .003), and ferritin (r = 0.715, P < .00001), and correlated negatively with platelet count (r = -0.422, P = .021), but not with other parameters, including tumor necrosis factor-alpha, IL-1 beta, IL-18, soluble Fas ligand and C-reactive protein. Multiple regression analysis indicated that the correlation of NSE with platelet count was independent of other correlations. Sequential NSE changes well reflected the clinical course of patients. Immunohistochemical staining revealed an appreciable number of NSE-positive histiocytes in bone marrow specimens with florid hemophagocytosis. These results suggest that the circulating NSE originated from macrophages stimulated with IFN-gamma/sIL-2R, and partly from the destruction of platelets. Serum NSE level may be a useful marker for predicting the disease progression of HLH.