Eighty-seven patients with cystic fibrosis were admitted to hospital with an acute exacerbation of pulmonary symptoms associated with isolation of Pseudomonas aeruginosa from sputum. The patients were randomly allocated to receive intravenously administered ceftazidime (250 mg/kg/day) and amikacin (33 mg/kg/day) alone or with inhaled amikacin (100 mg twice a day). Other aspects of the 2-week treatment were constant. The two therapy groups were comparable in all aspects. At the completion of therapy, the addition of aerosolized amikacin produced temporary eradication of P. aeruginosa in 70% of the patients, compared with 41% in the intravenous therapy only group (P less than 0.02). Suppression of P. aeruginosa in sputum cultures was correlated with the amikacin sputum concentrations. However, both regimens resulted in similar improvements in clinical, radiologic, laboratory, and pulmonary function measurements, and within 4 to 6 weeks most patients were recolonized with P. aeruginosa. There was no serious toxicity or adverse effect. In patients with cystic fibrosis, the addition of aerosol aminoglycoside to systemic antipseudomonal combination therapy is not clinically beneficial.