Dolichols are polyisoprenic molecule ubiquitously present in the lipid fraction of animal and plant tissues, discovered 40 years ago during experiments on the biosynthesis of ubiquinone. The molecular structure of dolichol comprises a sequence of unsaturated isoprenic units bearing a primary terminal hydroxyl group. The length of dolichyl chains depends on the species of the organism from which they are isolated. Mammalian dolichol generally is made up of 16 to 23 unsaturated isoprene units, and the terminal hydroxyl group may exist either free or esterified with fatty acids, phosphoric acid, and pyrophosphoric acid. In biological membranes, this linear polyisoprenoid compound may be located between the two leaflets of the lipid bilayer, close to the free end of the phospholipid fatty acid molecules. Metabolism and function of dolichol were largely unknown until recently. Synthesis of dolichol by the mevalonate pathway was demonstrated in vitro and in vivo in many tissues. The isoprenoid pyrophosphate intermediates are shared by the cholesterol, dolichol, and ubiquinone pathways, and treatment with drugs that block hydroxymethyl glutaryl coenzyme A reductase may significantly decrease their plasma and tissue levels. In humans, there is no apparent positive correlation between serum dolichol and tissue dolichol and age. In view of the total content of the body, half life of the total body dolichol, and dolichol content in the extracellular space, it was concluded that the dolichol in tissues probably derives from biosynthesis in those tissues and that relocation of dolichol via circulation cannot be prominent in vivo. The levels of dolichol in human serum have apparently no correlation to age or serum total cholesterol, and exhibit a linear correlation to high density lipoprotein cholesterols which may reflect the fact that the dolichols are associated with the high-density lipoprotein fraction. No enzymic pathways for dolichol degradation were described, but no case of dolichol-storage disease was reported. Shrinkage of tissue because of increased lysosomal degradation in the process of atrophy does not affect the dolichol content and concentration increases. Small quantities of dolichol that may be excreted into the urine at least in part is derived from the lysosomes of the excretory organ, and serum dolichol levels may be elevated in chronic cholestatic liver diseases. Recent evidence shows that phagocytosis may cause the degradation and disposal of the engulfed dolichol, possibly because of nonenzymatic free radical mediated decomposition. By means of a 1H nuclear magnetic resonance (NMR) analytical method, the hypothesis was substantiated that dolichol may act as a free-radical scavenger in the cell membranes and protect polyunsaturated fatty acids from peroxidation, and that it may undergo decomposition in the process. (PMID 15741281).