Due to the lack of prospective data, current treatment of male breast cancer (MBC) is based on information obtained from retrospective analysis or by extrapolation from studies on female patients. In this prospectively enrolled cohort study, we retrospectively examined the survival effect of tamoxifen in MBC patients. In this prospectively enrolled cohort study, 448 patients with MBC were treated between May 2009 and June 2018. The primary endpoint was disease-free survival (DFS). Between May 2009 and June 2018, 448 men with breast cancer were identified, with a median age at diagnosis of 69 years (range 27-96 years). The median follow-up was 39 months (range 3-89 months). Most tumours were larger than 20 mm; invasive ductal carcinoma was of no special histological type and with an intermediate grade of differentiation. Almost half of the men were diagnosed with positive axillary lymph nodes (43.5%). Hormone receptor (HR) positivity was observed in 98.4% of the patients. Notably, DFS among men who did not receive tamoxifen was significantly reduced as compared with those who underwent tamoxifen therapy (P = 0.002). The recurrence rate and mortality in the group of patients without and with tamoxifen treatment were 18.2% and 11.2%, respectively. The most common localisation of metastases was the bone. After adjustment for prognostic factors, we found that tamoxifen was found to reduce the recurrence rate by 68% (hazard ratio HR = 0.32; 95% confidence interval, CI: 0.14-0.74). Tamoxifen treatment was associated with improved DFS for MBC patients. DRKS00009536.