There is growing evidence that oxidized lipoproteins exist in vivo under certain conditions. Circumstantial evidence suggests lipoprotein oxidation may be an important event in atherosclerosis. Oxidized lipoproteins can be potent cytotoxins in vitro, although the manifestations of this cytoxicity in vivo are unknown. Current studies emphasize identifying the cytotoxic moieties, how they are formed and how they injure cells.