Antibiotics are administered to cystic fibrosis patients for chronic endobronchial infection complicated by frequent exacerbations. Agents active againstStaphylococcus aureus, Pseudomonas aeruginosaor both are administered. Serum antibiotic concentrations were measured in cystic fibrosis patients in an effort to optimise antibiotic dose and frequency. This led to the observation that cystic fibrosis subjects had (in general) a larger Vd and increased total body clearance of &bgr;-lactams and aminoglycosides than non-cystic fibrosis subjects. The larger Vd is mainly due to the increased amount of lean body mass per kg bodyweight, although increased tissue binding may also account for part of this. The increased total body clearance of &bgr;-lactams appears to be due to increased renal elimination, particularly tubular secretion. Decreased tubular reabsorption and increased non-renal clearance contribute to the increased total body clearance of metabolised &bgr;-lactams and aminoglycosides. However, the lack of concomitant controls in many studies make these generalisations tentative.The result of the apparent cystic fibrosis-specific differences is lower peak serum antibiotic concentrations, a smaller AUC, and a shorter elimination half-life than non-cystic fibrosis subjects. Since sputum (and bronchial mucosal) concentration is dependent on the peak serum concentration (and AUC), cystic fibrosis subjects require larger doses of most antibiotics more frequently. Newer quinolones may be an exception. Studies comparing the efficacy and safety of larger and more frequent antibiotic doses to conventional therapy are not available. Although it appears logical to mimic serum antibiotic concentrations found in non-cystic fibrosis subjects, the lack of information on the ideal sputum concentrationversustime curve should temper our enthusiasm for cystic fibrosis-specific dosage regimens.