Catechol estrogens (2- or 4-hydroxyestradiol and 2- or 4-hydroxyestrone) are chemically reactive estrogen metabolites that are O-methylated to less polar monomethyl ethers by catechol-O-methyltransferase, an enzyme present in many tissues such as the liver, kidney, brain, placenta, uterus, and mammary gland. In the present report, we review recent studies on the antitumorigenic and antiangiogenic effects of exogenously administered 2-methoxyestradiol in vitro and in vivo. We also discuss data that suggest that endogenous formation of 2-methoxyestradiol (and its 2-hydroxyestradiol precursor) may have a protective effect on estrogen-induced cancers in target organs. Although the molecular mechanism of action of 2-methoxyestradiol is not clear, we suggest that some unique effects of 2-methoxyestradiol may be mediated by a specific intracellular effector or receptor that is refractory to the parent hormone, estradiol. Additional research is needed to identify factors that regulate the metabolic formation and disposition of 2-methoxyestradiol in liver and in target cells and to evaluate the effects of modulating 2-methoxyestradiol formation on estrogen-induced carcinogenesis.