OBJECTIVE To determine whether gonadotropin-releasing hormone analogs (GnRH-a) initiated either in the luteal phase or in the early follicular phase immediately preceding menotropin will improve the fertilization, implantation, and pregnancy rates (PR) in all IVF patients, when compared with menotropins alone. METHODS In a prospective, controlled, randomized study we compared a pure follicle-stimulating hormone (FSH) human menopausal gonadotropin (hMG) protocol (group A = control) (n = 93 cycles) to two protocols in which GnRH-a pretreatment plus pure FSH and/or hMG was used in in vitro fertilization candidates. In group B (n = 64) GnRH-a was initiated during the luteal phase and in group C (n = 35) during the follicular phase. RESULTS We found (1) no differences in fertilization and implantation rates between the three protocols; (2) similar pregnancy rates per transfer when similar number of conceptus were transferred (A = 30%, B = 22%, C = 21%); (3) an increase of the number of oocytes obtained; and (4) a reduction in the cancellation rate with both GnRH-a protocols. CONCLUSIONS These findings suggest that there is no obvious superiority between the two GnRH-a protocols in the dosage schedule used and that the major advantage of GnRH-a over non-GnRH-a protocols is in decreasing the cancellation rate and increasing the number of oocytes and conceptus obtained. The follicular phase GnRH-a protocol required less hMG-pure FSH than the luteal phase GnRH-a protocol.