27-Hydroxycholesterol (27-HC), also known as (25R)-cholest-5-ene-3,26-diol or by its conventional name 26-hydroxycholesterol, is an oxygenated derivative of cholesterol and a major oxysterol in circulation. 27-Hydroxycholesterol is the product of the enzyme sterol 27-hydroxylase. The enzyme is critical for the degradation of the steroid side-chain and a genetic deficiency of the enzyme leads to reduced formation of bile acids in humans. There is a correlation between 27-hydroxycholesterol and cholesterol in the circulation, and females have lower levels of 27-hydroxycholesterol than males. A strong correlation is observed between circulating levels of 27-hydroxycholesterol and cholesterol, in both healthy subjects and subjects with hypercholesterolemia and documented atherosclerosis. 27-Hydroxycholesterol is metabolized by an oxysterol 7alpha-hydroxylase in the liver. Changes in the activity of this enzyme may lead to the accumulation of 27-hydroxycholesterol in the circulation. It has been reported that patients with a genetic deficiency of oxysterol 7alpha-hydroxylase in the liver had markedly increased levels of 27-hydroxycholesterol in the circulation. However, under normal conditions and in the absence of liver or kidney disease, changes in the levels of 27-hydroxycholesterol in the circulation are likely to be caused by changes in the rate of synthesis of these steroids rather than by the rate of metabolism. There are three possible explanations for the high concentrations of 27-hydroxycholesterol found in the circulation of three subjects with atherosclerosis: (1) increased expression of sterol 27-hydroxylase owing to a genetic factor or some other factor completely unrelated to atherosclerosis, (2) the extrahepatic sterol 27-hydroxylase may be up-regulated by circulating factors (e.g. cytokines) that are directly or indirectly related to the development of atherosclerosis, and (3) the high amounts of cholesterol accumulating in macrophages in some patients with atherosclerosis may result in an increased flux of 27-hydroxycholesterol from the macrophages to the circulation. Since there is a close relation between levels of cholesterol and 27-hydroxycholesterol in the circulation, the possibility must be considered that the flux of 27-hydroxycholesterol into the brain may be part of the yet unexplained link between hypercholesterolemia and Alzheimer's disease. 27-Hydroxysterol is the most dominant oxysterol in human atheromas where it may reflect a mechanism for eliminating excessive cholesterol, and thus have a protective role. Hypercholesterolemia and chronic low-grade immunological activation are pivotal in the development of atherosclerosis. However, the interconnections between these two factors are not well known. The CD40 system, as measured by the plasma level of soluble CD40 (sCD40), is associated with cholesterol metabolism in hypercholesterolemic patients. When combined, a decreased cholesterol synthesis rate and increased levels of 27-hydroxycholesterol may be a consequence of high levels of cellular cholesterol, and therefore be related to sCD40. However, sCD40 had no significant correlation with total plasma cholesterol. This suggests that the cellular cholesterol synthesis rate and 27-hydroxycholesterol production are more importantly linked with the plasma levels of sCD40 than total cholesterol.