A retrospective analysis of our in-vitro fertilization (IVF) and oocyte donation programmes was carried out in order to gain clinical knowledge of the factors involved in the aetiology of the endometriosis-associated infertility. Comparison between the IVF outcomes from 96 cycles in 78 patients with tubal infertility and from 96 cycles in 59 women with endometriosis indicates that endometriosis patients have a poor IVF outcome in terms of reduced pregnancy rate per cycle (P < 0.0004), reduced pregnancy rate per transfer (P < 0.002), and reduced implantation rate (P < 0.003). The analysis of patients undergoing oocyte donation for different reasons, including low response with or without endometriosis, showed that patients with this disease have the same chances of implantation and pregnancy as other recipients when the oocytes came from donors without known endometriosis. However, when the results of oocyte donation were classified according to the origin of the oocytes donated, patients who received embryos derived from endometriotic ovaries showed a significantly (P < 0.05) reduced implantation rate as compared to the remaining groups. Taken together, all these observations suggest that infertility in endometriosis patients may be related to alterations within the oocyte, which in turn result in embryos with decreased ability to implant.