Background: Eradication of Pseudomonas aeruginosa in patients with cystic fibrosis (CF) is possible if initiated early in the course of colonisation. To detect P aeruginosa as early as possible is therefore a major goal. This study was undertaken to validate a commercialised test for the detection of serum Pseudomonas antibodies in patients with CF.Methods: A representative cross sectional analysis of serum antibodies against three Pseudomonas antigens (alkaline protease, elastase, and exotoxin A) was performed in 183 patients with CF of mean age 16.7 years and FEV1 85.9% predicted. The results were correlated with microbiological results from the previous 2 years to calculate sensitivity, specificity, positive and negative predictive values. The following 2 years were assessed to determine prognostic predictive values.Results: A combination of all three tested antibodies yielded the best results with a sensitivity of 86%, specificity of 96%, and a positive predictive value of 97%. These values were higher if only patients in whom sputum cultures were available were considered (n = 76, sensitivity 95%, specificity 100%, positive predictive value 100%). The prognostic positive predictive value was high in intermittently infected patients (83%) but low in patients free of infection (33%), whereas the prognostic negative predictive value was high in patients free of infection (78%) and low in intermittently infected patients (58%).Conclusions: Regular determination of serum antibodies may be useful in CF patients with negative or intermittent but not with positive P aeruginosa status. A rise in antibody titres indicates probable infection and eradication treatment may be initiated even in the absence of microbiological detection of P aeruginosa.