The role of reactive oxygen species (ROS) in the pathogenesis of vascular diseases is well established, but few data exist on the mechanisms by which ROS induce endothelial cell (EC) death. We examined the conditions and the mechanisms by which oxidative stress induces EC death, using cultured confluent bovine aortic ECs exposed for 30 min to different concentrations of hydroxyl radicals (HO•) generated by hydrogen peroxide (H2O2) in the presence of 100 µM ferrous sulfate (FeSO4). Cell viability assays, Hoechst DNA staining, TUNEL (TDT-mediated dUTP-biotin nick end-labeling) analysis, agarose gel electrophoresis and annexin V assay were used to determine the effect of HO• on the viability of ECs, and to distinguish between apoptosis and necrosis. The results showed that at concentrations of up to 0.1 mM H2O2/FeSO4, the large majority of cells are viable, except for ~12.5% death, which occurs by apoptosis. At a concentration of 0.2 mM H2O2, the cell viability is reduced to 66%, while EC apoptosis remained at comparable values (14%). At high oxidative stress (0.5 mM H2O2), the cell viability was drastically reduced (~39%), and the prevalent form of death was necrosis; apoptosis accounted for only ~17%. Together, these data indicate that: (1) HO• induce EC death either by apoptosis or necrosis and (2) the mechanisms of EC death differ as a function of the concentration of HO. Thus, the same insult can cause apoptosis and/or necrosis, as a function of the intensity rather than the nature of the insult.