Recurrent and chronic pulmonary infection is still the major cause of morbidity and mortality in cystic fibrosis. Although respiratory viruses are responsible for some of the acute exacerbations of the pulmonary disease, bacteria, and in some patients Aspergillus fumigatus, are the most important pathogens. Staphylococcus aureus and Haemophilus influenzae are the most prevalent pathogens in cystic fibrosis of childhood, whereas Pseudomonas aeruginosa and in some centres also Pseudomonas cepacia predominate in older children and adult patients. The chronic Pseudomonas aeruginosa infection is peculiar, since it is predominantly an endobronchial infection in small bronchioles caused by mucoid, alginate producing strains which gradually lose most of the O-antigenic determinants of the lipopolysaccharide. Although P. aeruginosa produces a number of other toxins which may play a role initially, most if not all of the pathology is caused by immune complex mediated chronic inflammation. The bacteriological results of antipseudomonas chemotherapy are disappointing, as these bacteria are virtually never permanently eliminated. The clinical results of repeated maintenance chemotherapy every 3 months are, however, good, since it is possible to preserve lung function for years and keep the patients alive. Antiinflammatory treatment with steroids for years is used in some patients with benefit.