Several analyses in our infertility (IVF) and oocyte donation programs were carried out to gain clinical knowledge of the factors involved in the etiology of endometriosis-associated infertility. We first compared the IVF outcomes in women with tubal infertility and endometriosis. The results indicated that patients with endometriosis had a poorer IVF outcome in terms of reduced pregnancy rate per cycle, per transfer, and reduced implantation rate per embryo replaced. We then evaluated embryo development in vitro in women with and without endometriosis who underwent IVF and embryo replacement 72 hours after oocyte retrieval. We observed that compared to controls, patients with endometriosis had a significantly reduced number of blastomeres per embryo as well as an increased incidence of arrested embryos in vitro. In subsequent studies we compared fertility parameters in patients receiving donor oocytes. We noted that when donor oocytes came from patients without known endometriosis, embryo development and implantation rates were similar in patients with and without endometriosis. However, when the results of oocyte donation were classified according to the nature of the oocytes donated, patients who received embryos derived from oocytes from women with endometriotic ovaries showed a significantly reduced implantation rate compared to the controls. Taken together, these observations suggest that IVF in patients with endometriosis may be related to alterations within the oocyte, which, in turn, result in embryos of lower quality with a reduced ability to implant.