Background: Many patients with cystic fibrosis (CF) suffer from allergic disease, which can complicate treatment of CF lung disease. Interleukin (IL)-4 and IL-13 have been shown to be important mediators in allergic disease. Objective: To investigate the role of IL-4 and IL-13 in allergic and non-allergic CF patients. Methods: Expression of IL-4 and IL-13 mRNA was investigated in peripheral blood mononuclear cells (PBM) of seven CF patients with allergy, of six patients without allergy and of nine healthy subjects as well as in BAL cells of four patients and of all controls. PBM from six patients were incubated with recombinant human IL-13 or human antiIL-13 antibody without and with LPS stimulation and TNFα levels were measured by ELISA. Results: IL-13 mRNA expression was increased in allergic and non-allergic patients compared to controls. No significant difference in IL-4 expression could be found between patients and controls. Addition of IL-13 decreased TNFα in PBM culture supernatants. Conclusion: Our data suggest that IL-13 rather than IL-4 might play an important role in both allergic and non-allergic CF patients. IL-13 might also compromise host defence by decreasing TNFα production.