To assess the efficacy of gonadotropin-releasing hormone analog (GnRHa) as an adjuvant in controlled ovarian stimulation in assisted conception programs, 114 infertile patients, who were treated by in vitro fertilization and embryo transfer (n = 61) or tubal embryo transfer (n = 53), were randomized sequentially to receive ovarian stimulation according to two protocols. In protocol 1 (n = 57), long-acting GnRHa (D-Trp-6-LHRH) microcapsules were administered intramuscularly at menstruation and ovarian stimulation using follicle-stimulating hormone (FSH) and human menopausal gonadotropin (hMG) was started 2 to 3 weeks later when the pituitary was completely suppressed. In protocol 2 (n = 57), patients received FSH and hMG from day 3 of the cycle without GnRHa pre-treatment. We found that premature luteinization did not occur in patients treated with protocol 1, and the number of cycles cancelled was also decreased. The days of ovarian stimulation and the amount of hMG required to achieve adequate follicular development were significantly higher in protocol 1 than that in protocol 2. Similarly, the mean serum estradiol levels on the day of human chorionic gonadotropin administration, number of large follicles (mean diameter greater than 10 mm), number of oocytes recovered and number of embryos obtained were also significantly higher in patients treated with protocol 1. The data suggest that the use of D-Trp-6-LHRH as an adjuvant in ovarian stimulation is associated with a lower incidence of cycle cancellation and an improvement in ovarian response in assisted conception programs.