Cholesterol and bile acid metabolism and vitamin B12 were studied in 19 patients with heterozygous familial hypercholesterolemia in an 8-yr follow-up (3-12 yr after ileal bypass operation), and in 11 unoperated controls. Absorption of cholesterol and vitamin B12 were decreased by the operation, but improved slowly, and at 8 yr cholesterol absorption was normal. Cholesterol excretion as fecal neutral steroids was not increased by the operation, and at 8 yr the flux of endogenous cholesterol to the gut was similar in the operated and control patients. Cholesterol absorption was positively correlated with mouth to anus transit time in the unoperated patients. Fecal bile acid excretion was increased immediately after the operation and continued to increase even after the second postoperative year. In the operated patients fecal excretion of fat, water, and dry matter were positively correlated with fecal bile acid excretion. Our study suggests that adaptive changes occur slowly after ileal bypass, resulting in gradual normalization of cholesterol absorption, despite continuing bile acid malabsorption, and that the intestinal transit time is related to steroid absorption even under physiologic conditions.