The pathogenesis of endometriosis, an estrogen-dependent disease, remains elusive. Early work proposed the initiation of the disease is due to “retrograde menstruation” and this has been widely accepted. However, to date, the mechanism of endometriotic cell proliferation is still unexplained. Oxidative stress has been implicated as contributing to cell growth. The expression of heme oxygenase 1 (HO-1) is upregulated by oxidative stress and is capable of protecting cells against oxidant-mediated injury. HO-1 has been suggested as being involved in the mechanism of cell proliferation, and estradiol has also been implicated in proliferative events. In the present study, we determined the effects of oxidative stress and estradiol on endometriotic cell proliferation and on the expression of HO-1 using immortalized human endometriotic epithelial cells (12-z). Cells were treated with the oxidants H₂O₂ (10 µM) or menadione (25 µM), or the lipid peroxidation product acrolein (35 µM) for 48 hours and the proliferation of cells was determined by MTT [(3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay. The results show that H₂O₂, menadione and acrolein significantly induced cell proliferation at 1 µM, 20 µM and 25 µM respectively, showing that H₂O₂ is the most potent compound. Cells were also pre-treated with estradiol at 10^-9 M for 24 hours followed by the addition of compounds at similar concentrations for 24 hours. Interestingly, in the presence of estradiol at 10^-8 M, the cells that were treated with lower concentrations of compounds showed a significant increase in proliferation, but this was not the case in the presence of estradiol at 10^-9 M. In contrast, at high concentrations, all compounds caused significant growth inhibition. The expression of HO-1 mRNA in treated cells was determined by Quantitative Real Time-Polymerase Chain Reaction (Q-PCR). A significant up-regulation of HO-1 mRNA was observed in all the treated cells and further increased in the presence of estradiol at 10^-9 M. These results indicate that oxidative stress may contribute to the proliferation of endometriotic cells. In addition, there may be some interplay with estradiol to further increase cell proliferation. Expression results indicate that estradiol may be involved in the regulation of the HO-1. The up-regulation of HO-1 mRNA during proliferation indicates that this enzyme could be used as a reliable marker in the pathogenesis of endometriosis.