Most cystic fibrosis (CF) patients with chronic Pseudomonas aeruginosa lung infection have a persistent acute type lung inflammation dominated by polymorphonuclear neutrophils (PMN) and a pronounced antibody response against P. aeruginosa. We speculated whether this immune response in CF is of the Th2 type and whether a change to a Th1 type immune response could improve the prognosis. Therefore, we studied 14 CF patients with (CF +P) and 14 CF patients without (CF −P) chronic P. aeruginosa lung infection. The specific production of interferon-gamma (IFN-γ) and interleukin-4 (IL-4) by peripheral blood mononuclear cells was determined. Cells from CF +P patients had lower IFN-γ (p<0.05) and higher IL-4 (p<0.005) production as compared to cells from CF −P patients. Furthermore, a positive correlation between IFN-γ production and lung function was found (FVC: Rho=0.637; p<0.03; FEV1: Rho=0.524; p<0.07). We conclude that a Th2 type immune response is most frequent in CF patients with chronic P. aeruginosa lung infection, and the patients with a Th1-dominated immune response had the best lung function. The clinical implication is that a change to a Th1 type immune response might improve the prognosis in these patients.