An aminoglycoside in association with beta-lactam antibiotic are usually the most efficient treatment for Pseudomonas aeruginosa infections in cystic fibrosis patients. Tobramycin has the lower MICs than the other aminoglycosides to Pseudomonas aeruginosa. The aim of the work was to compare pharmacokinetics of tobramycin after once daily (15 mg/kg/day; 11 patients) or thrice daily dose (5 mg/kg/day; 9 patients) in combination ceftazidime (CAZ 200 mg/day in 3 inj. IVD) in sputum and sera for two weeks. No statistical difference in the serum concentration obtained in each group of patients was observed between the first and the 14th day. Serum concentrations were three fold higher when tobramycin was administered in once daily dose. Low through concentrations were quickly obtained, but they were slightly higher after thrice daily doses. Bronchial concentrations were 2 to 2.5 superior and near the critical concentration of tobramycin. The clinical efficacy were comparable in the two regimens.