Cystic fibrosis is now the most common cause of chronic obstructive pulmonary disease (COPD) and of pancreatic insufficiency in the first three decades of life in the United States. In this report we describe 75 patients with cystic fibrosis aged 18 to 47 years and review another 232 cases reported in the literature. All of these 307 patients had elevated sweat chloride and sodium levels, which proved excellent discriminants for cystic fibrosis even in patients in the older age group. COPD, present in 97 per cent, was the major cause of morbidity and mortality, and differed from COPD of other etiologies. The progressive downhill course in these patients was punctuated by recurrent symptomatic exacerbations of chronic bacterial bronchitis caused by Pseudomonas aeruginosa and Staphyloccocus aureus, and terminated in pulmonary insufficiency, cor pulmonale and death. COPD was complicated by minor hemoptysis in 60 per cent, massive hemoptysis in 7 per cent and pneumothorax in 16 per cent, problems rare in children. Sinusitis was present in all those examined roentgenographically, and 48 per cent had nasal polyposis. Pancreatic insufficiency was present in 95 per cent of the patients, but in contrast to younger patients it was seldom symptomatic although steatorrhea and azotorrhea were still massive. Intussusception and meconium ileus equivalent (fecal accumulation) are frequent in adults (21 per cent) but rare in children, and they require immediate diagnostic and therapeutic intervention with enemas of diatrizoate sodium. Glycosuria, biliary cirrhosis, cholelithiasis and aspermia were among other complications. Height and weight were usually within the lower limits of normal, but 17 per cent of the men were above 180 cm in height and 7 per cent were overweight. Therefore, a high index of suspicion is needed to make the diagnosis, because older patients with cystic fibrosis may look quite well. © 1978.