Serum levels of interleukin-16 (IL-16) were measured to investigate its role in the pathophysiology of hemophagocytic lymphohistiocytosis (HLH). Serum IL-16 levels in patients with acute HLH were significantly higher than those in healthy controls and patients with infectious mononucleosis. They returned to normal levels in the convalescent phase of the disease. In contrast to serum interferon-gamma (IFN-gamma) levels, serum IL-16 levels showed a gradual decrease over the course of the disease. Serum IL-16 levels showed a significant positive correlation with serum levels of soluble IL-2 receptor, IFN-gamma, and interleukin-18, body temperature, and serum lactic dehydrogenase (LDH) levels. An increase in IL-16 mRNA expression was detected in the liver of an HLH patient. These results suggest that IL-16 plays an important role in the pathophysiology of HLH by TH1 cell recruitment and activation at organs with inflammation.