Seven healthy volunteers and 3 hospitalized patients were given the same diet for 6 days. The mean intake of cholesterol was 359 mg/day. There was a large difference in the fecal amount of cholesterol and coprostanol amongthe subjects. The ratio of coprostanol/cholesterol ranged from 0.01 to 4.27. The subjects whose feces contained smaller amountof coprostanol excreted larger amounts of β-sitosterol and crude fiber, suggesting that the various activities of intestinal flora were lower than the other subjects.The difference in the activity of intestinal flora to convert cholesterol into coprostanol was ascertained by incubating the feces anaerobically with egg yolk in vitro. It was also demonstrated that cholesterol was synthesized de novo and the amount corresponded to 0.1-0.6 g/day. The amounts of sterols in the feces of the other 17 hospitalized patients were also determined and there was an inverse relationship between the serum cholesterol and coprostanol/cholesterol ratio in the feces. These results suggested that the conversion of cholesterol into coprostanol by the intestinal flora might have an important role in the regulation of serum cholesterol level.