Catechol estrogens, the hydroxylated metabolites of 17beta-estradiol (E2), have been considered to be implicated in estrogen-induced carcinogenesis. 2-Hydroxyestradiol (2-OHE2), a major oxidized metabolite of E2 formed preferentially by cytochrome P-450 1A1, reacts with DNA to form stable adducts and exerts genotoxicity. 2-OHE2 can be oxidized to quinone, which is accompanied by generation of reactive oxygen species (ROS). In the present study, 2-OHE2 induced strand scission in phiX174 phage DNA and oxidative base modifications in calf thymus DNA in the presence of cupric ion. In cultured human mammary epithelial (MCF-10A) cells, 2-OHE2 treatment produced ROS accumulation, 8-oxo-7,8-dihydroxy-2'-deoxyguanosine formation, cytotoxicity, and disruption of mitochondrial transmembrane potential, all of which were prevented by N-acetylcysteine. These findings, taken together, suggest that 2-OHE2-induced oxidative DNA damage and apoptosis in MCF-10A cells might be mediated by ROS generated via the redox cycling of this catechol estrogen.