20 patients with cystic fibrosis and chronic bronchopulmonary infection due to Pseudomonas aeruginosa entered a randomized cross-over study comparing ceftazidime (150 mg/kg body weight/24 h) plus tobramycin (10 mg/kg body weight/24 h) to ceftazidime alone (150 mg/kg body weight/24 h), both given intravenously for 2 weeks. 17 patients completed the study; both treatment regimens improved lung function and decreased the WBC count. No difference in clinical efficacy was found between the treatments. Pulmonary function returned to pre-treatment levels 3 months later with no difference between the treatments. No changes were seen in minimal inhibitory concentrations during treatment. None of the patients developed hypersensitivity or experienced serious adverse reactions to the drugs.