<p>Tamoxifen (TAM) is a non-steroidal anti-estrogen used widely in the treatment of breast cancer. However, long-term use of TAM increases the risk of endometrial cancer. To clarify the molecular mechanism of carcinogenesis, we examined DNA damage by TAM and its metabolites, 4-hydroxytamoxifen (4-OH-TAM), 3-hydroxytamoxifen (3-OH-TAM) and 3,4-dihydroxytamoxifen (3,4-diOH-TAM). 3,4-DiOH-TAM caused oxidative damage to ³²P-5’-end-labeled DNA fragments in the presence of Cu(II) and NADH, while TAM, 4-OH-TAM and 3-OH-TAM did not. The HOMO level of 3,4-diOH-TAM was calculated to be the highest among TAM metabolites. 3,4-DiOH-TAM induced DNA cleavage at cytosine and guanine residues of the ACG sequence complementary to codon 273, a well-known hotspot in the <i>p53</i> gene. Catalase and bathocuproine inhibited DNA damage induced by 3,4-diOH-TAM in the presence of Cu(II) and NADH, suggesting the involvement of H₂O₂ and Cu(I). 3,4-DiOH-TAM with Cu(II) induced 8-oxodG formation in calf thymus DNA, which was markedly enhanced in the presence of NADH. Oxidative DNA damage induced by a TAM metabolite 3,4-diOH-TAM may contribute to the initiation of endometrial carcinogenesis. Furthermore, the reported proliferative effect of TAM on endometrial cells is involved in the promotion stage of tumor development. Taken together, it is suggested that both mechanisms may play roles in endometrial carcinogenesis.</p>