Various studies have demonstrated pronounced systemic IgG response to Pseudomonas aeruginosa (PA) infection in cystic fibrosis (CF). However, antibody response to serotype-specific lipopolysaccharides (LPS) has never been studied. ELISA for detection of IgG antibodies to LPS of nine PA-serotypes and to toxin A were performed with serum of 78 CF patients. Anti-LPS profiles of antibodies were confirmed by SDS-PAGE and immunoblotting techniques. The most frequent PA-serotypes found were immunotypes (IT) IT-1 and IT-2, and Habs-3 and Habs-4. Ten patients without PA colonization showed no detectable antibody titers. In patients with chronic PA colonization (n=46), these antibody titers were significantly (p<0.005) higher than in patients with intermittent PA colonization (n=22). Mean serum antibody titers to LPS of PA IT-1, IT-2, Habs-3, and Habs-4 correlated with duration of PA colonization and with disease severity. Subclass analysis of anti-LPS antibodies revealed elevated levels for all four IgG subclasses and for IgA1. The IgG antibodies to LPS of PA proved to be protective in a murine burn wound sepsis model. We conclude that anti-LPS antibodies to specific PA serotypes in serum may be a sensitive measure of severity and prognosis of CF. Patients with CF show adequate functional immune response to LPS of PA, and it is possible that vaccination against PA before colonization could induce protective immunity.