The discovery of a family of hormonal steroids esterified with fatty acid has raised questions concerning their physiologic role. Because of their water-insolubility these compounds are present in the circulation only as components of lipoprotein particles. Current evidence supports the hypothesis that estrogen esterification is catalyzed by lecithin:cholesterol acyltransfearse associated with HDL. In addition, recent results indicate that estradiol esters are transferred from HDL to LDL particles in a cholesteryl ester transfer protein (CETP)-associated process. The studies now focus on the various possible physiologic roles proposed for these hormone derivatives, (1) functioning as fat-soluble antioxidants incorporated in lipoproteins rendering protection against oxidation of these particles, (2) providing a mechanism for hormonal storage in lipoproteins and fat tissues, (3) providing a novel hormone transport system using lipoprotein as carriers and lipoprotein receptors for entry into cells. Quantitative methods of determination of estradiol fatty acid esters in human body fluids have been developed. Preliminary studies suggest that diet-derived plant estrogens may also form fat-soluble derivatives which become incorporated in lipoproteins.