Quantitative fecal bile acids (in milligrams per 24 hours per square meter of body-surface area) in 24 children with untreated pancreatic insufficiency secondary to cystic fibrosis were increased (743.2 ± 55.3 — mean ± S.E.) whereas those in 12 cases of celiac disease and in seven with chronic nonspecific diarrhea did not differ from values in 18 controls (110.0 ± 11.0). Bile acid loss in cystic fibrosis was comparable to that of six infants after ileal resection (957.7 ± 195.1). A large increase (p<0.01) in fecal bile acids occurred when pancreatic enzyme therapy was withdrawn in five patients with cystic fibrosis. These data and the good correlation (r = 0.78) found in cystic fibrosis and not in celiac disease between fecal fat and bile acids suggest that unhydrolyzed fat or other products of maldigestion may interfere with bile acid absorption. The daily fecal bile acid sequestration of 1.9 g and 3.7 g found in two cases of cystic fibrosis studied during pancreatic supplementation indicates that a definite interruption of the bile acid enterohepatic circulation may occur in this disease. (N Engl J Med 289:1001–1005, 1973)