Several retrospective analyses of our in-vitro fertilization (IVF) and oocyte donation programmes have been carried out in an attempt to gain clinical knowledge of the factors implied in the aetiology of endometriosis-associated infertility. In a first approach, a comparison was made of the IVF outcome between 96 cycles in 78 patients with tubal infertility and 96 more cycles in 59 women with endometriosis. The results indicate that endometriosis patients had a poor IVF outcome in terms of a reduced pregnancy rate per cycle (P < 0.0004), a reduced pregnancy rate per transfer (P < 0.002) and a reduced implantation rate per embryo replaced (P < 0.003). In a second study, we addressed the analysis of patients undergoing oocyte donation. The results showed that patients with this disease have the same chances of implantation and embryo development in vivo as other recipients when the oocytes come from donors without known endometriosis. However, when the results of oocyte donation were classified according to the nature of the oocytes donated, patients who received embryos derived from endometriotic ovaries showed a significantly (P < 0.05) reduced ability to implant compared with the remaining groups. In a third approach, we evaluated embryo development in vitro when women with and without endometriosis underwent IVF and embryo replacement 72 h after oocyte retrieval. We observed a significantly (P < 0.04) reduced number of blastomeres in embryos from endometriosis patients compared with controls, as well as an increased (P < 0.05) incidence of arrested embryos in vitro. Taken together, these observations suggest that infertility in endometriosis patients may be related to alterations within the oocyte which, in turn, result in embryos of lower quality, as demonstrated in our IVF programme, and a lower ability to implant, as shown in the oocyte donation model.