Lobar atelectasis occurred in 30 (4.1 per cent) and segmental atelectasis occurred in 6 (0.8 per cent) of 728 patients with cystic fibrosis. The right lung was involved in all 11 patients less than 5 years of age and in 18 of 26 episodes in 19 older patients. Five of the younger patients died within 5 years of the initial episode. Bronchoscopy (with or without local lavage) performed in addition to intensified medical therapy did not appear to improve the outcome. Future studies of therapeutic measures (including bronchoscopy) for treatment of atelectasis complicating cystic fibrosis must include appropriate control patients who receive only medical therapy.