To determine the effect of ovarian endometriomas on in vitro fertilization (IVF) outcome, two groups of patients were studied. Group I consisted of seven patients with ovarian endometriomas and severe pelvic adhesions treated for a total of 12 cycles. Group II patients consisted of eight patients with hydrosalpinges and comparable pelvic adhesions treated for a total of 27 cycles. There were no differences in the number of days required for stimulation or in the serum estradiol levels attained between the two groups. Group I patients were noted to have significantly fewer preovulatory follicles (1.42 vs 3.33, P less than 0.005), cycles with fertilization (28 vs 84%, P less than 0.005), and embryos transferred (0.78 vs 2.56, P = 0.01) than Group II patients. Three pregnancies occurred in Group II, while there were no conceptions among Group I patients. This study suggests that the presence of an ovarian endometrioma(s) has an adverse effect on IVF outcome and suggests that patients with ovarian endometriomas should have them removed prior to undergoing IVF.