We measured serum interferon-gamma-inducible protein 10 (IP-10) and monokine induced by gamma interferon (MIG) levels to investigate the role of these molecules in the pathophysiology of haemophagocytic lymphohistiocytosis (HLH). Serum IP-10 and MIG levels were significantly increased in patients with active HLH compared with those of healthy controls. Serum MIG levels decreased gradually during the course of disease in a patient who recovered without therapy. On the other hand, rapid reduction of MIG and IP-10 levels was observed after chemotherapy in a patient with severe HLH. IP-10 and MIG mRNA expression was enhanced in liver and spleen, and IP-10 mRNA expression was enhanced in bone marrow in the patients, suggesting activated macrophages that infiltrated in these organs as one of the main producers of these cytokines. Serum IP-10 and MIG levels showed a significant correlation with serum IFN-gamma levels. In addition, these chemokines had a significant correlation with fever and serum LDH levels, which are clinical indicators of disease activity of HLH. These results suggest that IP-10 and MIG which are produced by activated macrophages by the stimulation of IFN-gamma, play an important role in the pathophysiology of HLH, by recruitment of activated Th1 cells into the tissues or organs.