Ten adolescent and young adults with cystic fibrosis (CF) have had well-documented recurrent attacks of acute pancreatitis. The diagnosis of CF in each patient was delayed because they did not have pancreatic insufficiency. The diagnosis of CF was documented by the typical pulmonary involvement and elevated sweat sodium and chloride levels in all cases and a positive family history in six of the ten patients. Two patients were diagnosed as having acute pancreatitis before the diagnosis of CF was made, thus indicating that acute pancreatitis may be the presenting complaint in the young adult with CF. The diagnosis of acute pancreatitis was based on the presence of severe abdominal pain, usually with vomiting, tenderness in the mid-epigastrium, elevated serum and urinary amylase and serum lipase. Attacks were precipitated by fatty meals, alcohol ingestion; postcholecystectomy and tetracycline administration. In some patients no precipitating event could be elicited. Intravenous secretin-pancreozymin stimulation tests revealed a diminished bicarbonate secretion with little effect on the secretion of the zymogen enzymes. A mild attack of pancreatitis occurred after secretin-pancreozymin stimulation. The endocrine pancreatic function tested in four patients was normal as revealed by the glucose tolerance tests and determinations of serum insulin, growth hormone and free fatty acid. Transduodenal pancreatograms were performed in three patients; one showed a normal pancreatic duct, one showed duct obstruction and in the third patient a beady type of narrowing was found. The selenomethionine Se 75 uptake of the pancreas was noted only in the head of the pancreas. This suggests that loss of function occurs initially to a greater extent in the tail and body of the pancreas. Three patients died and showed characteristic lesions of CF.