The antimicrobial prescribing practices of 26 physicians from the U.K. and Eire who care for patients with cystic fibrosis (CF) were assessed by postal questionnaire. Our main aim was to delineate divergent practices which may reveal a need for controlled prospective studies. For first-line intravenous (i.v.) therapy of acute exacerbations associated with Pseudomonas aeruginosa, 20 physicians (76.9%) combine a penicillin derivative with an aminoglycoside, in contrast to five (19.2%) who regularly use i.v. monotherapy with ceftazidime and one who combines i.v. ceftazidime with an aminoglycoside. When i.v. therapy is considered inappropriate, oral ciprofloxacin is sometimes used by all clinicians and oral broad spectrum agents are used in addition by 13, chloramphenicol being prescribed most often. Excluding allergy, the most important factor influencing choice of agents by 19 (73.1%) physicians is the most recent sputum susceptibility results. For maintenance therapy, 21 prescribe oral anti-staphylococcal agents if Staphylococcus aureus is isolated; of these, eight do so only if lung function deteriorates, nine after repeated isolation and four after first isolation of S. aureus. The remaining five physicians give anti-staphylococcal drugs to all patients once the diagnosis of CF is made. For maintenance of lung function in patients with persistent P. aeruginosa, all physicians used nebulized antibiotics, the indications for which vary between units. There was general concordance in the therapy of exacerbations associated with P. aeruginosa, whereas the use of agents to maintain lung function is more varied. We suggest that prospective studies address practices which vary greatly, such as the route, the duration and the timing of initiating antibiotic therapy given to maintain lung function.