The elevated level of circulating estradiol increases the risk of breast tumor development. To gain further insight into mechanisms involved in their actions, we investigated the molecular mechanisms of 4-hydroxyestradiol (4-OHE₂) to initiate and/or promote abnormal cell growth, and of α- or γ-tocopherol to inhibit this process. MCF-10A, human breast epithelial cells were incubated with 0.1 µM 4-OHE₂, either with or without 30 µM tocopherols for 96 h. 4-OHE₂ caused the accumulation of intracellular ROS, while cellular GSH/GSSG ratio and MnSOD protein levels were decreased, indicating that there was an oxidative burden. 4-OHE₂ treatment also changed the levels of DNA repair proteins, BRCA1 and PARP-1. γ-Tocopherol suppressed the 4-OHE₂-induced increases in ROS, GSH/GSSG ratio, and MnSOD protein expression, while α-tocopherol up-regulated BRCA1 and PARP-1 protein expression. In conclusion, 4-OHE₂ increases oxidative stress reducing the level of proteins related to DNA repair. Tocopherols suppressed oxidative stress by scavenging ROS or up-regulating DNA repair elements.