The annual mortality rate of cystic fibrosis patients with chronic Pseudomonas aeruginosa lung infection at the Danish CF-centre ranged from 10 to 20% in the years 1970-1975. In this period the patients received antipseudomonal chemotherapy only during acute exacerbations of infection. From 1976 99 patients acquired chronic P. aeruginosa infection and were given regular and intensive antipseudomonal treatment 3-4 times per year. The patients were followed for 612 patient-years; 7 died and the 10-year survival rate after onset of P. aeruginosa infection was 90% +/- 4%. The annual mortality rate is now 1-2%. Although precipitating antibodies against P. aeruginosa increased significantly, pulmonary function did not deteriorate with duration of infection. Cross-infection between patients caused an increased incidence of chronic P. aeruginosa infection which was reduced by hygienic measures.