A double blind controlled trial of Becotide (beclomethasone diproprionate) inhalations was carried out for treating cystic fibrosis patients with chronic P. aeruginosa lung infection to determine its efficacy and safety. The aim of the treatment was to diminish the inflammatory response in the lungs of these patients, a response which is initiated by an allergic type III reaction. Pulmonary inflammation was evaluated by measurements of proteolytic activity, albumin concentration and immune complex activity in the sputum sol-phase before, during, and after the 16 weeks the trial lasted. 26 cystic fibrosis patients participated (13 received Becotide and 13 placebo) and the results showed that local steroids have no effect on the inflammatory response in the lungs of cystic fibrosis patients with chronic P. aeruginosa lung infection. No adverse effects were demonstrated. There was, however, a significant increase in the inflammatory parameters for all 26 cystic fibrosis patients when the trial period was over, compared to the insidious pulmonary destruction which takes place in the lungs of these patients, and it corresponded to a significant decrease (p less than 0.05) in forced vital capacity which took place at the same time. Therefore, chronic P. aeruginosa lung infection in these patients should be treated as efficiently as possible.