The literature suggests that the results of in-vitro fertilization (IVF) for patients with endometriosis depend on the stage of the disease, and that patients with severe endometriosis have a higher failure rate. Miscarriage is said to be more prevalent in women treated for endometriosis. In the study reported here, 140 patients with endometriosis underwent 182 cycles of IVF using gonadotrophin-releasing hormone analogues (GnRHa). Patients with endometriosis only were allocated to one group (group 4). The results were compared with those of three other groups of patients undergoing the same treatment within the same period. Group 1 consisted of couples with male factor only (45 cycles), group 2, couples with unexplained infertility (196 cycles) and group 3, couples with a tubal factor only (1139 cycles). The mean age of the patients, mean number of human menopausal gonadotrophin (HMG) ampoules administered, oestradiol concentration on the day of human chorionic gonadotrophin administration, number of days of HMG, mean number of oocytes retrieved and retrieval rate were not significantly different. The fertilization rate was significantly lower in group 1; no difference was observed in the other three groups. The mean number of normally fertilized embryos was not significantly different. The number of transferred embryos in each cycle and the implantation rates were similar in the four groups. The overall pregnancy rate per transfer was 39% in group 1, 48% in group 2, 45% in group 3 and 40% in group 4. The miscarriage rate for all patients was zero for group 1, 3.9% for group 2, 3.9% for group 3, and, remarkably (and unlike all previous reports), none of the pregnant patients with endometriosis miscarried. The endometriosis group was divided according to the American Fertility Society (AFS) revised classification (1985) and the results analysed separately. No difference in the results was found between patients with AFS stage I-II or AFS stage III-IV disease. Our results suggest that the presence and severity of endometriosis does not alter the outcome of IVF, and we did not find any evidence of an increased incidence of miscarriage.