Purpose The aim of this study was to evaluate the long-term effect of 2 years of adjuvant tamoxifen compared with no systemic treatment (control) in premenopausal patients with breast cancer over different time periods through long-term (amp;gt; 25 years) follow-up. Patients and Methods Premenopausal patients with primary breast cancer (N = 564) were randomly assigned to 2 years of tamoxifen (n = 276) or no systemic treatment (n = 288). Data regarding date and cause of death were obtained from the Swedish Cause of Death Register. End points were cumulative mortality (CM) and cumulative breast cancer-related mortality (CBCM). The median follow-up for the 250 patients still alive in April 2014 was 26.3 years (range, 22.7 to 29.7 years). Results In patients with estrogen receptor-positive tumors (n = 362), tamoxifen was associated with a marginal reduction in CM (hazard ratio [HR], 0.77; 95% CI, 0.58 to 1.03; P = .075) and a significant reduction in CBCM (HR, 0.73; 95% CI, 0.53 to 0.99; P = .046). The effect seemed to vary over time (CM years 0 to 5: HR, 1.05; 95% CI, 0.64 to 1.73; years amp;gt;5 to 15: HR, 0.58; 95% CI, 0.37 to 0.91; and after 15 years: HR, 0.82; 95% CI, 0.48 to 1.42; CBCM years 0 to 5: HR, 1.09; 95% CI, 0.65 to 1.82; years amp;gt;5 to 15: HR, 0.53; 95% CI, 0.33 to 0.86; and after 15 years: HR, 0.72; 95% CI, 0.36 to 1.44). Conclusion Two years of adjuvant tamoxifen resulted in a long-term survival benefit in premenopausal patients with estrogen receptor-positive primary breast cancer. (C) 2016 by American Society of Clinical Oncology. Creative Commons Attribution Non-Commercial No Derivatives 4.0 License: https://creativecommons.org/licenses/by-nc-nd/4.0/