Data comparing the efficacy of monthly and trimonthly formulations of LHRH agonists are lacking. The aim of this study was to compare the effects of monthly goserelin and trimonthly leuprolide on estradiol levels. A total of 79 early breast cancer patients receiving LHRH agonists for at least 6 months were enrolled in the study. Serum estradiol, FSH and LH levels were measured before drug injection and at the one-month follow-up visit. Thirty-eight patients were treated with goserelin, and 41 patients were treated with leuprolide. Patient characteristics and histopathological variables did not differ between the groups. A comparison of the mean hormone levels between the two groups revealed no significant differences in FSH or estradiol levels (p = 0.143 and p = 0.683, respectively), but the median LH level was higher in the leuprolide group (p = 0.025). Among the patients who did not receive chemotherapy, LH levels were higher in the leuprolide arm (p = 0.028). Additionally, FSH levels were significantly higher in the patients over 40 years old (p = 0.02) and in those with tumours harbouring cERB-B2 receptor (p = 0.05) in the leuprolide group. Three patients (7.9 %) in the goserelin and five patients (12.2 %) in the leuprolide group failed to achieve postmenopausal estradiol levels (p = 0.707). The effects of monthly goserelin and trimonthly leuprolide on estradiol levels did not differ significantly. Further research is required to interpret the variable effects on gonadotropins in each subgroup and the relationship between LHRH agonists and survival.