Approximately one-third of breast cancers lack estrogen receptor α (ERα) because of the hypermethylation of the CpG island in the receptor's promoter. These tumors are associated with poorer histological differentiation, a higher growth fraction, are rarely responsive to endocrine therapy and have a worse clinical outcome. Thus, re-expression of ERα in ERα-negative breast cancers may restore the sensitivity of antiestrogen therapy. The ERα-negative breast cancer cell line MDA-MB-435s was treated with different concentrations of arsenic trioxide (As2O3). MS-PCR was used to detect the change in the methylation status of ERα. RT-PCR, immunohistochemistry and Western blot analyses were used to detect changes in the mRNA and protein expression of DNA methyl-transferase-1 (DNMT1) and ERα. Cell proliferation was examined using the MTT assay. A xenograft model in nude mice was used to further examine the results we observed in vitro. The ERα gene was demethylated after As2O3 treatment of MDA-MB-435s cells. RT-PCR, immunohistochemistry and Western blot analyses revealed that DNMT1 expression was inhibited and ERα was re-expressed in a concentration-dependent manner after As2O3 treatment. The MTT assay showed that cell proliferation was significantly suppressed after exposure to different concentrations of As2O3. Addition of tamoxifen (TAM) further suppressed levels of cell proliferation. In vivo, the xenograft tumor volumes of As2O3-treated mice were smaller than those observed in untreated and TAM-treated mice. Treatment with a combination of As2O3+TAM resulted in further suppression. As2O3 can act as a demethylation agent to restore ERα expression in ERα-negative breast cancer cells and re-sensitize these cells to endocrine therapy in vitro and in vivo.