This study compared the efficacy and safety of a 3-monthly 10.8-mg depot goserelin (Zoladex(TM)) injection with the current 3.6 mg monthly dose in pre-menopausal Japanese women with estrogen receptor-positive (ER+) early breast cancer. This was a multicenter, open-label, randomized study. Primary endpoint was a non-inferiority analysis (10.8/3.6 mg) of the area under the concentration-time curve (AUC) of estradiol (E(2)) over the first 24 weeks. Secondary endpoints included E(2) and follicle-stimulating hormone (FSH) concentrations, menstruation, and safety and tolerability. In total, 170 patients were randomized to receive goserelin 10.8 mg every 3 months (n = 86) or 3.6 mg every month (n = 84). Mean AUCs for E(2) were similar between treatment groups (18.32 and 18.95 pg/ml·week for goserelin 10.8 and 3.6 mg, respectively). AUC ratio was 0.974 (95% confidence interval, 0.80, 1.19), indicating non-inferiority for goserelin 10.8 mg. Serum E(2) and FSH remained suppressed throughout the study and no patient experienced menses after week 16. No clinically important differences in safety and tolerability were observed between the two groups. In terms of E(2) suppression, 3-monthly goserelin 10.8 mg was non-inferior to monthly goserelin 3.6 mg in pre-menopausal women with ER+ breast cancer.