Data from the literature and the authors' experiences were used to review aspects of antibiotic therapy of patients with cystic fibrosis; attention was paid to in vitro antimicrobial susceptibility tests and assessment of therapy directed against mucoid Pseudomonas aeruginosa. The heterogeneity of P. aeruginosa within single sputa with respect to antibiotic susceptibility is stressed. Quantitative viable counts of bacteria based on an analysis of homogenised sputum is recommended. The mode of in vivo growth of mucoid P. aeruginosa is discussed to explain the survival of hypersusceptible P. aeruginosa in vivo, and the clinical benefit observed in the absence of a significant reduction of the pathogen. The value of ceftazidime in the treatment of exacerbations due to Haemophilus influenzae is emphasised. The social benefits from oral administration of ciprofloxacin also emphasises that the patient's quality of life must also be considered.