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Purpose: To gauge the effects of treatment practices on prognosis for older patients with HER2-positive early breast cancer, particularly to determine whether adjuvant trastuzumab alone can offer benefit over no adjuvant therapy. This is a prospective cohort study which accompanies the RESPECT that is a randomized-controlled trial (RCT). Methods: Patients who declined the RCT were treated based on the physician's discretion. We studied the 1) trastuzumab-plus-chemotherapy group, 2) trastuzumab-monotherapy group, and 3) non-trastuzumab group (no therapy or anticancer therapy without trastuzumab). The primary endpoint was disease-free survival (DFS), which was compared using the propensity-score method. Relapse-free survival (RFS) and health-related quality of life (HRQoL) were assessed. Results: We enrolled 123 patients aged over 70 years (median: 74.5). Treatment categories were: trastuzumab-plus-chemotherapy group (n = 36, 30%), trastuzumab-monotherapy group (n = 52, 43%), and non-trastuzumab group (n = 32, 27%). The 3-year DFS was 96.7% in trastuzumab-plus-chemotherapy group, 89.2% in trastuzumab-monotherapy group, and 82.5% in non-trastuzumab group. DFS in non-trastuzumab group was lower than in trastuzumab-plus-chemotherapy and trastuzumab-monotherapy groups (propensity-adjusted hazard ratio; HR: 3.29; 95% CI: 1.15–9.39; P = 0.026). The RFS in non-trastuzumab group was lower than in trastuzumab-plus-chemotherapy and trastuzumab-monotherapy groups (propensity-adjusted HR = 7.80; 95% CI: 2.32–26.2, P < 0.0001). There were no significant intergroup differences in the proportions of patients showing HRQoL deterioration at 36 months (P = 0.717). Conclusion: Trastuzumab-treated patients had better prognoses than patients not treated with trastuzumab without deterioration of HRQoL. Trastuzumab monotherapy could be considered for older patients who reject chemotherapy.