In cystic fibrosis infection is localized to the bronchi and self-perpetuated by a disorder of mucociliary clearance. At a certain stage of the disease infection is caused by Pseudomonas aeruginosa (PA), and the patient cannot get rid of it as cystic fibrosis reduces his defence mechanisms against this organism, a naturally weak pathogen. The presence and significance of persistent bronchial PA infection result from complex phenomena which have been summarized as "an opportunistic organism in a host whose defences have been very peculiarly compromised". The toxicity of PA and the inappropriate inflammatory response of the host concur to create permanent alveolar lesions which are gradually spreading out. The effectiveness of antibiotics depends on the organism and its resistance, on pharmacokinetic factors bringing the drug in situ and on local conditions of redox, physico-chemical environment and nutrition. Other factors intervene more roughly, including patient's age, liver impairment, lung anatomy (excluded areas), general malnutrition and vitamin deficiencies. Thus, the part played by other treatments in the antibiotic therapy of cystic fibrosis can no longer be ignored, just as it is no longer sensible to limit the use of antibiotics to obvious infections. To evaluate the effectiveness of antibiotics is not easy, but it can be demonstrated beyond any doubt when the above mentioned factors are taken into account. The use of antibiotics creates practical problems relating to their cost, their dosage, their side-effects, the adequacy of therapeutic infrastructures and, above all, it raises the question of short- and long-term changes in microbial ecology.