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Hydroxytyrosol found in extra virgin olive oil strongly inhibited low-density lipoprotein oxidation stimulated by 2,2‘-azobis(2-amidinopropane) hydrochloride (AAPH), suggesting the ability to scavenge the AAPH-derived peroxyl radicals. Hydroxytyrosol inhibited iron-dependent phospholipid liposome peroxidation at low concentrations (IC50 = 50 ± 1.3 μM). In similar experiments, the calculated, IC50 values for other antioxidants compared are 1.5 ± 0.05 μM (carnosol), 2.25 ± 0.08 μM (carnosic acid), 65 ± 2.6 μM (Trolox C), and 250 ± 10 μM (vitamin E). Hydroxytyrosol and ascorbate reduced copper(II) ions to their copper(I) prooxidant form, but this was not reflected by their abilities to induce oxidative DNA damage in the complex copper−phenanthroline. Only high, nonphysiological, millimolar concentrations of pure hydroxytyrosol weakly stimulated copper-dependent chemical modification to DNA bases. The prooxidant (redox actions on metal ions) concentrations in vitro may never be achieved in vivo (following consumption of extra virgin olive oil). Thus, hydroxytyrosol may represent a useful diet-derived antioxidant depending on its bioavailability. Keywords: Extra virgin olive oil; hydroxytyrosol; low-density lipoprotein; rosemary; vitamin E; lipid peroxidation; plant antioxidants; DNA damage