Cultured endothelial cells from bovine aorta were exposed to oxidized low density lipoprotein and examined by electron microscopy. The endothelial cells contracted slightly and the intercellular junctions became unclear. Some osmiophilic material increased in the cytoplasm. The oxidized low density lipoprotein appears to injure endothelial cells and thereby plays a causative role in atherogenesis and thrombogenesis.