From April, 1980 to December, 1981 ten children aged from 2 to 14 years presenting with cystic fibrosis were admitted to hospital for exacerbation of their chronic bronchial infection. Mucous Pseudomonas aeruginosa was present in sputum. Seven of the 10 strains isolated were susceptible to azlocillin and 3 were classified as intermediate. Eight children were treated with azlocillin alone in doses of 200-300 mg/kg/day and two with combined azlocillin 300 mg/kg/day and amikacin 16 and 23 mg/kg/day respectively. In both groups the antibiotics were administered 8-hourly by short (30 min) intravenous infusions and the duration of treatment ranged from 8 to 21 days (mean 14 days). Both drugs were well tolerated. Antibacterial activity was assessed as "cure" when Pseudomonas could not be isolated in sputum for at least 2 weeks after the end of the treatment, as "relapse" when that organism reappeared in sputum within the same period of time, and as "failure" when it persisted in sputum. On this account, among the 8 children treated with azlocillin alone 3 were cured, one relapsed and 4 failed. One of the two children treated with the azlocillin-amikacin association was cured, the other failed. Clinical results correlated roughly with antibacterial activity. Five distinct improvements were observed: 2 were associated with bacteriological cure, 2 with transient eradication followed by relapse and 1 occurred although the responsible organism persisted in bronchial secretions. Two children showed poor clinical results; 2 with failure and 1 with bacteriological cure. In two other children treatment was ineffective both clinically and bacteriologically. This study confirms that high parenteral doses of azlocillin have a beneficial effect on exacerbations with Ps. aeruginosa of chronic bronchial infection in cystic fibrosis. Clinical improvement usually correlates with antibacterial activity when the organism is eliminated, even temporarily, from bronchial secretions. The synergistic azlocillin-aminoglycoside association should probably be recommended, at least to reduce the risk of emergence of resistant strains.