Cross-infection with Pseudomonas aeruginosa and Pseudomonas cepacia has been shown sometimes to occur between cystic fibrosis (CF) patients in some CF centres, in some summer camps and during some social contacts between CF patients. Cohort isolation and improved hygienic precautions, however, have successfully been employed in some CF centres resulting in a decrease or elimination of cross-infection. Chronic P. aeruginosa infection is in most CF patients preceded by a period of intermittent colonization. Early aggressive chemotherapy (oral ciprofloxacin and nebulized colistin for 3 weeks) every time P. aeruginosa is detected in sputum has significantly decreased the incidence of new chronic infection in CF patients. Chronic P. aeruginosa infection can be treated by "maintenance chemotherapy" (= chronic suppressive chemotherapy). The principle is to restore lung function repeatedly by regular 2-week courses of intensive intravenous treatment every 3 months in the CF centre and adding daily inhalations of colistin between the courses, and sometimes also by giving oral ciprofloxacin during these intervals to unstable patients. Eradication of P. cepacia infection is virtually never obtained by antibiotic treatment with, e.g., ceftazidime and tobramycin or co-trimoxazole. Chronic suppression with doxycyclin or co-trimoxazole may give rise to some improvement in the clinical symptoms.