Twenty-one patients with cystic fibrosis and chronic Pseudomonas lung infection were treated at random with ceftazidime, 150 mg/kg/day, or with piperacillin, 300 mg/kg/day, and tobramycin, 10 and more mg/kg/day for 14 days. On admission and at discharge, body weight, erythrocyte sedimentation rate, white blood cell count, and differential were determined. Pulmonary function analysis and chest X-rays were also obtained on both occasions as was sputum bacteriology. After hospitalization, the patients were followed in the outpatient department for 14-26 months. Both treatments were associated with significant improvement in most of the parameters that were studied, but neither treatment was superior.