The enzyme lecithin cholesterol acyl transferase is responsible for the synthesis of most of the cholesteryl esters in plasma, and therefore plays a key role in lipoprotein metabolism. The relationship between the structure and function of lecithin cholesterol acyl transferase has been extensively studied in the past years, and new data appeared in 1999 documenting the substrate specificity and physiological role of lecithin cholesterol acyl transferase. The discovery of natural mutants, together with the proposal of a three-dimensional model for the enzyme, has provided new tools to unravel the function of specific residues of lecithin cholesterol acyl transferase. The use of transgenic animals and the production of knock-out lecithin cholesterol acyl transferase mice has further contributed to the understanding of the lecithin cholesterol acyl transferase 'in vivo' function. Evidence for a protective role of lecithin cholesterol acyl transferase against the development of atherosclerosis through the hydrolysis of oxidized lipids was recently proposed. Lecithin cholesterol acyl transferase patterns in several pathologies were further clarified. These newer developments are reviewed here.