25-Hydroxycholesterol (Figure 1) is a minor oxysterol that may be formed in different types of tissues by a specific enzyme not belonging to the cytochrome P-450 family (7). The mode of regulation of this enzyme has not been defined. 25-Hydroxycholesterol is known to be converted into normal bile acids in the liver, although less efficiently than other compounds, such as 7α-hydroxycholesterol. Very recently it was shown that one of the major oxysterols in the circulation, 4β-hydroxycholesterol (Figure 1), is formed from cholesterol in the liver and possibly also in the intestine by the cytochrome P-450 species CYP3A4 (8). While the metabolic end products of 4β-hydroxycholesterol have not yet been defined, the existence of 4β-hydroxylated bile acids in humans suggests that this oxysterol can be metabolized in the liver.