Some studies have shown that long-term gonadotropin-releasing hormone (GnRH) agonist administration before in vitro fertilization/intracytoplasmic sperm in infertile women with endometriosis or adenomyosis significantly increases the chances of pregnancy. We were interested in whether long-term GnRH agonist pretreatment could improve pregnancy outcomes in adenomyosis patients undergoing frozen embryo transfer (FET) after preparation of the endometrium with hormone replacement therapy (HRT). Totally, 339 patients with adenomyosis were included in this retrospective study, 194 received long-term GnRH agonist plus HRT (down-regulation + HRT) and 145 received HRT. There were no differences between the groups in characteristic such as age, body mass index, duration or cause of infertility, serum CA-125 level and basal hormone levels. On the day of progesterone administration, mean endometrial thickness and serum progesterone level were significantly greater in HRT patients. Mean score and number of embryos transferred showed no differences. In down regulation + HRT group, clinical pregnancy, implantation and ongoing pregnancy rates were 51.35%, 32.56% and 48.91%, respectively, significantly higher than that of HRT group (24.83%, 16.07% and 21.38%, respectively). So, we concluded that in FET, long-term GnRH agonist pretreatment significantly improved pregnancy outcomes in patients with adenomyosis.