Cystic fibrosis is the most common fatal inherited disease of Caucasians. At present, cystic fibrosis accounts for most cases of chronic progressive pulmonary disease and for many other clinical features in the first three decades of life. Thus, it is a challenge to both pediatricians and internists, particularly chest physicians. The diagnosis is based on the triad of chronic obstructive pulmonary disease, pancreatic insufficiency, and increased levels of electrolytes in the sweat. The cardinal test for confirmation of the diagnosis is the "sweat test," which is an excellent discriminant for cystic fibrosis, even in adults. Ancillary features of cystic fibrosis may be of diagnostic assistance (eg, nasal polyposis, Pseudomonas aeruginosa in sputum, azoospermia, and others). Treatment of the pulmonary disease must be emphasized. Choice of antibiotics should be based on the results of sputum culture, but P aeruginosa is the most common pathogen. Removal of secretions by regular postural drainage and percussion is an integral part of the program. Pneumothorax, massive hemoptysis, cor pulmonale, and other complications may be encountered. Sinusitis is almost universal, and nasal polyposis is frequently present. Pancreatic insufficiency occurs in over 80 percent of the patients with cystic fibrosis and may result in intestinal malabsorption. Massive salt loss through the sweat in hot weather, a distinctive type of biliary cirrhosis without jaundice, gallbladder abnormalities, cholelithiasis, and diabetes mellitus also may be found. Of special importance are intestinal obstructive complications (meconium ileus in newborn infants with cystic fibrosis and intestinal obstruction due to fecal accumulation or intussusception in adults). Azoospermia is present in 95 percent of men and there is reduced fertility in women; however, pregnancy does occur in cystic fibrosis. This chronic and ultimately fatal disease produces a predictable set of psychosocial complications.