Early in 1983 an epidemic of a Pseudomonas aeruginosa resistant to aminoglycosides, carbenicillin, ureidopenicillins, ceftazidime, cefsulodin and imipenem occurred in a cystic fibrosis centre. Most of the epidemic could be attributed to a specific nosocomial strain by means of O-grouping and phage-typing. This strain was present in the centre at a low frequency in 1973 and developed resistance during courses of chemotherapy. The epidemic was stopped by isolating patients with the resistant strains. Restrictive and selective use of antibiotics have not been sufficient to eradicate the resistant strains, which persist in 42% of the patients. The extensive use of the third generation cephalosporins in the clinic is probably responsible for inducing and selecting for the resistant strains. Clustering of patients in the centre has facilitated the spread. First-line use of older beta-lactam antibiotics, close bacteriological monitoring and prompt isolation of patients with resistant strains are recommended.