The fatty acid composition of bile secreted into the duodenum in the first 10 min after an intravenous (i.v.) injection of Boots secretin (2 CHRu kg-1) has been analysed by gas liquid chromatography in 11 healthy volunteers, 8 patients without pancreatic disease, 27 patients with exocrine pancreatic disease who had not altered their diet substantially (acute pancreatitis 8; chronic pancreatitis 16; cancer 3) and 11 patients with exocrine pancreatic disease on low fat intakes (40 g/day) for at least 6 months. The mean values for total fatty acid outputs (after back transformation of the logged data) were significantly higher in each subgroup of patients with pancreatic disease on their usual diets (acute 134, chronic 189, cancer 235 mg) than in the two subgroups of controls (30 and 55 mg), due to significant increases in the outputs of every fatty acid, C16:0 through to C22:5. This finding, which was usually not apparent in patients with pancreatic disease on low-fat diets, may reflect the combined influence of dietary fat intakes and hepatic enzyme induction. Comparison of the fatty acid outputs in endoscopically collected bile and duodenal juice after separate injections of secretin three hours apart indicate that: (a) analysis of duodenal juice within 10 min of stimulation by Boots secretin provides valuable information on the composition of hepatic bile; (b) the increased phospholipid output in the untreated patients is due to hypersecretion and does not merely represent a 'washout' phenomenon.