A great number of basic and clinical studies have demonstrated that inflammatory cytokines play an important role in development and progress of heart failure. However, there is limited information about allergic cytokine interleukin-13 (IL-13). The inflammatory responses mediated by allergic cytokines can cause significant morbidity and mortality when they become chronic. Therefore, we elucidated the role of IL-13 in the pathophysiology of chronic heart failure. We measured plasma IL-13 levels by enzyme-linked immunosorbent assay in 110 patients with chronic heart failure and 20 control subjects. Plasma IL-13 levels were increased in heart failure patients, compared with the controls, in association with NYHA functional class. In addition, IL-13 levels were correlated positively with plasma levels of brain natriuretic peptide and C-reactive protein, and negatively with left ventricular ejection fraction. Plasma IL-13 levels may be useful for evaluating disease severity in chronic heart failure.