Fourteen children with cystic fibrosis and pulmonary lung infection due to Pseudomonas aeruginosa were treated with piperacillin (300 mg/kg/day) alone or piperacillin and tobramycin (7 mg/kg/day) iv. The outcome with respect to clinical state, chest X-ray and lung function tests was better with combination therapy than with piperacillin alone. Bacteriological response was the same with both regimens: leucocyte content of the sputum decreased, non-mucoid Ps. aeruginosa strains were eliminated, but mucoid strains were only suppressed (11 children). Peak serum levels of piperacillin averaged 102 mg/l, the overall serum elimination was 0.75 h and the mean sputum concentrations ranged from 1.07 to 2.2 mg/l. Peak serum levels of tobramycin averaged 5.15 mg/l, the half life was 1.25 h and the mean sputum concentrations ranged from 0.57 to 0.68 mg/l. The clearance of piperacillin and tobramycin was increased significantly. Drug-resistance did not develop during therapy.