In patients with cystic fibrosis (CF) and chronic colonisation with Pseudomonas aeruginosa, specific anti-pseudomonal IgG and IgA, as well as serum immunoreactive protein C, WBC and differential count, ESR, pulmonary function and chest radiograph score were determined before and after a 2 week intravenous course of anti-pseudomonal antibiotics in 32 cases of acute exacerbation of pulmonary infection. Specific anti-pseudomonal IgA but not specific anti-pseudomonal IgG decreased significantly after treatment. Log of anti-pseudomonal IgA but not log anti-pseudomonal IgG correlated well with disease severity as assessed by the Brasfield chest radiograph score (r 0.57), forced expiratory volume in 1 s (r 0.6) as well as C-reactive protein (r 0.62). CONCLUSION: Specific anti-pseudomonal IgA may be a better parameter than specific IgG in the follow up of lung infection in patients with CF, probably because it more closely reflects ongoing endobronchial infection, the major pathology in CF lungs.