This invention provides novel assays that are prognostic and/or diagnostic for atherosclerosis or risk of atherosclerosis. It was discovered that high density lipoprotein (HDL) or components thereof can prevent the oxidation of lipids (e.g. lipids present in LDLs) and can also repair (reduce) already oxidized lipids and thereby reduce the inflammatory response associated with and characteristic of atherosclerotic plaque formation. Moreover it was a discovery of this invention that individuals vary in the ability of their HDL to afford such protection. Thus an assay of HDL protective and/or repair activity provides a highly effective assay for risk of atherosclerosis and its associated pathologies and such assays are provided herein.