The short-term effects of high carbohydrate diets of normal foods either high or low in dietary fiber on fecal steroids and fiber was assessed in eight healthy young men. Each subject consumed in random order for 4 days a diet containing 59 g (high fiber) or 21 g (low fiber) neutral detergent fiber. After a 9-day rest period he consumed the other diet. Analysis of random fecal samples during their usual diet and after 4 days of each experimental diet showed an increased in primary bile acids from less than 4 to 32% of total bile acids, and a decreases of coprostanol from 76% (control diet) or 64% (low fiber diet) to 45% of total neutral sterol after the high fiber diet. Fecal fiber concentration doubled after the high fiber diet. We conclude that 4 days of high fiber diet is sufficient to cause a large increase in primary and decrease in secondary fecal steroids. Such changes have implications for prevention of arteriosclerosis and cancer of the colon.