In 5 children with cystic fibrosis, 13 courses of lower respiratory infections due to Pseudomonas aeruginosa were treated with azlocillin, 100-200 mg/kg body weight intravenously every 8 h for 10-15 days. The clinical course during azlocillin treatment was more favourable than had been the case previously in the same patients when treated with combinations of carbenicillin and aminoglycosides. No side effects ascribable to azlocillin were observed, although one allergic reaction occurred, but this was probably elicited by another allergen. Upon repeated courses of treatment, the minimum inhibitory concentration of the infecting organisms increased steadily against both azlocillin and carbenicillin. It is concluded that azlocillin represents an important alternative in the treatment of lower respiratory tract infections due to P. aeruginosa in patients with cystic fibrosis.