Bile acid loss (mg/m2 24h) in the stools of 43 cystic fibrosis (CF) children with pancreatic insufficiency was 751-1 +/- 48-3, while that of six without clinical evidence of pancreatic disease (133-4 +/- 15-9) did not differ from values in 25 controls (109-8 +/- 9-8). There was a good correlation between the degree of bile acid (BA) and fat sequestration. Concomitant changes in bile acid and fat loss were observed in the one group of six patients studied on and off pancreatic enzymes as well as in a second group of seven children treated with pancreatic supplements and maintained on a normal diet followed by a low fat diet supplemented with medium chain triglycerides. Administration of NA bicarbonate led to a significant decrease in fat loss (15-8 +/- 2-7 leads to 10-3 +/- 1-9) without any simultaneous change in bile acid excretion (533-1 +/- 58-3 leads to 500-4 +/- 58-6). Qualitative bile acid patterns in controls, in infants after an ileal resection, and in patients with CF or with coeliac disease showed that the percentage of primary BA followed closely the total amount excreted except in situations where antibiotics were administered. The exact mechanism for the increased loss of BA in CF is unknown. It is found in all age groups and is related to the presence and degree of pancreatic insufficiency. The possibility that unhydrolysed triglycerides may interfere with the intestinal absorption of bile acid needs further confirmation.