PURPOSE The addition of ovarian function suppression (OFS) for 5 years to tamoxifen (TAM) for treatment of premenopausal patients with breast cancer after completion of chemotherapy has beneficial effects on diseasefree survival (DFS). This study evaluated the efficacy of adding 2 years of OFS to TAM in patients with hormone receptor-positive breast cancer who remain in a premenopausal state or resume ovarian function after chemotherapy. PATIENTS AND METHODS We enrolled 1,483 premenopausal women (age ≤ 45 years) with estrogen receptor- positive breast cancer treated with definitive surgery after completing adjuvant or neoadjuvant chemotherapy. Ovarian function was assessed every 6 months for 2 years since enrollment on the basis of follicular-stimulating hormone levels and vaginal bleeding history. If ovarian function was confirmed to be premenopausal at each visit, the patient was randomly assigned to complete 5 years of TAM alone (TAM-only) group or 5 years of TAM with OFS for 2 years that involved monthly goserelin administration (TAM + OFS) group. DFS was defined from the time of enrollment to the time of the first event. RESULTS A total of 1,293 patients were randomly assigned, and 1,282 patients were eligible for analysis. The estimated 5-year DFS rate was 91.1% in the TAM + OFS group and 87.5% in the TAM-only group (hazard ratio, 0.69; 95% CI, 0.48 to 0.97; P = .033). The estimated 5-year overall survival rate was 99.4% in the TAM + OFS group and 97.8% in the TAM-only group (hazard ratio, 0.31; 95% CI, 0.10 to 0.94; P = .029). CONCLUSION The addition of 2 years of OFS to TAM significantly improved DFS compared with TAM alone in patients who remained premenopausal or resumed ovarian function after chemotherapy.