Background: The combined analysis of the Suppression of Ovarian Function Trial (SOFT) and Tamoxifen and Exemestane Trial (TEXT) trials have shown significant survival benefit of adding ovarian function suppression (OFS) with GnRH agonists (GnRHa) to aromatase inhibitors (AI) versus tamoxifen in high-risk premenopausal patients (pts) with estrogen receptor (ER)-positive breast cancer (BC). In both studies, GnRHa was administered every 4 weeks. However, the efficacy of 3-monthly GnRHa plus AI has not been well studied. We conducted a retrospective study designed to assess the efficacy of OFS with 3-montlhy compared with monthly goserelin, associated with AI, in premenopausal pts with ER-positive BC. Methods: Medical records of premenopausal pts with ER-positive BC who received OFS with 10.8mg 3-montlhy goserelin plus AI or 3.6mg monthly goserelin plus AI, between June/2013 and January/2023 were reviewed. Patient characteristics, such as age, body mass index (BMI) and prior chemotherapy were analyzed. Estradiol (E2) levels were measured by gas chromatography tandem mass spectrometry. Pts with at least one E2 level above 2.72 pg/ml were considered to have ineffective OFS. Analysis of E2 measurements was divided into periods: < 3, 3 to 6, 6 to 9, 9 to 12, 12 to 18, 18 to 24 and > 24 months. E2 levels greater than 2.72 pg/ml were evaluated at each timepoint. Results: A total of 88 pts were included. 27 (30.7%) received monthly goserelin and 61 (69.3%) 3-monthly goserelin. Patient characteristics were well balanced between the groups. Relevant demographic data are shown in table. For E2 analysis, 20 (22.7%) pts had at least one measurement greater than 2.72 pg/ml, 11 (40.7%) in the monthly group and 9 (14.8%) in the 3-monthly group (p = 0.007). The percentages of E2 measurements ≥ 2.72 pg/ml, in each time period, were 0%, 40%, 12.5%, 16.7%, 33.3%, 50% and 50% in monthly group versus 8%, 4.2%, 10%, 14.3%, 4.8%, 8.3% and 5.6%, in 3-monthly group, respectively. Conclusions: Our study demonstrated that more pts who underwent 3-monthly goserelin had E2 levels below the defined threshold of 2.72 pg/ml compared with monthly goserelin, when associated with AI, with statistical significance. These findings suggest that 3-monthly goserelin may result in better efficacy in OFS when comparing with monthly and provide greater convenience for pts.