A family of extremely reactive electrophiles, isolevuglandins (isoLGs), is generated in vivo by free radical-induced lipid oxidation and rearrangement of endoperoxide intermediates of the isoprostane pathway. Protein adducts of two different oxidized lipids, isoLGE₂ and iso[4]LGE₂, and the corresponding autoantibodies are present in human blood. Western blot analysis of a polyacrylamide gel electrophoresis gel detects several immunoreactive plasma proteins. Only a minor fraction of the isoLG–protein modifications is associated with low density lipoprotein since mean levels were decreased only 20–22% by immunoprecipitation of apolipoprotein B (apoB). Mean levels of both isoLGE₂ and iso[4]LGE₂–protein adducts in plasma from patients with atherosclerosis (AS) (n=16) or end-stage renal disease (RD) (n=8) are about twice those in healthy individuals (n=25). These elevated levels are not related to variations in age, total cholesterol or apoB. A linear correlation (r=0.79) between plasma isoLGE₂ and iso[4]LGE₂–protein adduct levels in all 49 individuals is consistent with a common free radical-induced mechanism for the production of both oxidized lipids in vivo. The correlation is even stronger (r=0.86) for patients with AS or RD. That isoLG–protein adduct levels are more strongly correlated with disease than are total cholesterol or apoB suggests an independent defect that results in an abnormally high level of oxidative injury associated with AS and RD.