The establishment of the conversion of cholesterol to coprostanol by the intestinal microflora was followed in 25 healthy Swedish children by gas-chromatographic analysis of fecal samples taken at 0, 1, 3, 6, 9, 12, 15, 18, 21, and 24 months of age. In 15 children, the microbial conversion of cholesterol to coprostanol started during the second half of the first year of life. During the second year of life, a sequestered distribution of the conversion rate could be distinguished. At the end of the study, 13 of the children were stable high converters, 3 were stable low converters, and 6 were unstable converters while the conversion pattern of three children could not be determined. The initiation of the establishment of conversion was delayed by breast-feeding, and only one child had coprostanol in feces while still being breast-fed. Those children exclusively breast-fed for more than 4 months or weaned after 6 months of age had significantly lower conversion rates at 12 months of age than did the other children (p < 0.05). Once conversion started, previous breast-feeding tended to result in a stable, later high rate of conversion of cholesterol. Between 6 and 15 months of age, the conversion rate increased with age, but after standardization for the period of exposure to foods other than breast milk, no age factor could be identified.