Based on the findings of several reports that have shown an increased plasma level of di-(2-ethylhexyl) phthalate (DEHP) in women with endometriosis, the present study was designed to evaluate whether in vitro treatment with DEHP can increase viability of endometrial cells. Utilizing 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide assay, fluorescent activated cell sorter analysis, and microscopic evaluation after Hoechst staining, we revealed that in vitro treatment with DEHP leads to increased viability of Ishikawa cells as well as endometrial stromal cells in serum-free condition and following exposure to hydrogen peroxide, which suggests that exposure to phthalate might play a role in the establishment of endometriosis.