Interleukin 18 (IL-18) is a recently described proinflammatory cytokine. In mouse models it has been shown to play a key role in the development of liver injury. IL-18 binding protein (IL-18BP) is a naturally occurring antagonist of IL-18. In this study we investigated whether IL-18/IL-18BP levels are altered in patients with chronic liver disease (CLD). We measured IL-18 and IL-18BP plasma levels in 153 patients with CLD and 41 healthy controls by a specific ELISA. Plasma levels of IL-18 were significantly higher in CLD patients than in healthy controls. Cirrhotics had higher levels than noncirrhotics. IL-18 levels increased with disease progression. IL-18BP plasma levels paralled the increase of IL-18 with disease progression, except in stage Child C cirrhosis. IL-18 and IL-18BP levels were elevated independent of the etiology of CLD. IL-18 and IL-18BP correlated with laboratory parameters of inflammation and liver injury. Plasma levels of IL-18 and its antagonist, IL-18BP, are elevated in CLD and correlate with severity of disease. IL-18BP may not be sufficient to counteract the overwhelming proinflammatory response in end stage liver disease.