Interleukin-18 (IL-18) is a recently discovered proinflammatory cytokine which plays a pivotal role in T helper 1 (Th1) responses. IL-18 is produced by macrophage-like cells, and inappropriate IL-18 production has been known to be involved in immunological disturbances. Schizophrenia is a common disease whose pathogenesis is still unclear; however, an activation of the inflammatory response system, including the Th1 cytokine response, may be related to the pathophysiology of schizophrenia. We measured the serum IL-18 levels of 66 schizophrenics and age- and sex-matched control subjects by using an ELISA assay. We found significantly increased serum IL-18 levels in the schizophrenic patients (P=0.0002). This finding supports the hypothesis that immune activation is involved in the pathophysiology of schizophrenia.