Xenoestrogens are part of a group of agents termed endocrine disruptors because of their capacity to perturb normal hormonal actions. It has been suggested that xenoestrogens may contribute to the development of hormone-dependent cancers, such as breast and endometrial cancers. Bisphenol A (BPA) is polymerized to manufacture polycarbonate plastic and epoxy resins. Human exposure occurs when BPA leaches from plastic-lined food and beverage cans. In the present work we are aiming to determine if BPA has carcinogenic properties by using an in vitro system. For this purpose the human breast epithelial cells MCF-10F were treated with 10-3M to 10-9M BPA continuously for two weeks. The MCF-10F cells treated with 10-3M and 10-4M BPA died, indicating that these concentrations were toxic for the human breast epithelial cells. The cells treated with 10-5M to 10-9M BPA were evaluated for formation of solid masses in collagen and invasion capacity, both phenotypes that are indicators of cell transformation. MCF-10F treated with 10-5M, 10-6M, 10-7M, 10-8M and 10-9M BPA formed a high percentage of solid masses (34.6%, 20%, 42.4%, 31.8% and 32.2%, respectively). Ten passages after BPA treatments, the invasive capacities of the cells were evaluated using Boyden chambers. The invasion capacity was lower in the cells treated at high concentrations of BPA (10-5M and 10-6M), and there was an increased invasion for the cells treated at low BPA concentration (10-9M) although, in all the cases the differences were not significant to the controls. Expression and DNA methylation analyses were performed with the cells treated with 10-5M and 10-6M BPA. We found that these cells showed an increased expression of BRCA1, BARD1, CtIP, RAD51 and BRCC3, all genes involved in DNA repair, and downregulation of PDCD5 and BCL2L11 (also known as BIM), both involved in apoptosis. The upregulation of CtIP was related to hypomethylation of the promoter/exon1 of this gene. Furthermore, BPA induced silencing of BCL2L11 by hypermethylation. This is the first demonstration that BPA induces neoplastic transformation of breast epithelial cells and that aberrant DNA methylation of genes involved in DNA repair and apoptosis could be involved in the initiation of the neoplastic process.