Background: Adjuvant endocrine therapy (ET) is the standard of care for Estrogen Receptor (ER)-positive early-stage breast cancer. However, many patients suffer from adverse events associated with ET. The prognosis of patients with T1a/bN0 breast cancer with ER-positive and human epidermal growth factor 2 (HER2)-negative is excellent, and it is not clear whether adjuvant ET should be recommended. Here we evaluate the effect of adjuvant ET for ER-positive and HER2-negative T1a/bN0M0 breast cancer. Methods: This study was a multicenter, retrospective cohort study that evaluated adjuvant ET for patients with ER-positive and HER2-negative T1a/bN0M0 breast cancer. Patients who underwent surgery between 2008 and 2012 in 47 institutions of the Japan Clinical Oncology Group (JCOG) breast cancer group were eligible. We analyzed the cumulative incidence of distant metastasis as primary endpoint using the Gray test model and time to event endpoints (distant disease free survival and overall survival) using log-rank test between patients treated with and without adjuvant ET. We also used Gray test model for the analysis of the cumulative incidence of ipsilateral and contralateral breast cancer. Predictive factors were assessed using the Fine-Gray model and the Cox proportional hazards model. Results: The median follow-up period was 9.2 years. Of 4758 eligible patients (1202 T1a [25.3%] and 3556 T1b [74.7%] diseases), 3991 (83.9%) received adjuvant ET. The 9-year cumulative incidence of distant metastasis was 1.5% with ET and 2.6% without ET (adjusted subdistribution hazard ratio [sHR], 0.54; 95% CI, 0.32-0.93). In the multivariate analysis, the independent risk factors of distant metastasis were no ET, mastectomy, high grade, and lymphatic invasion. The 9-year overall survival was 97.0% and 94.4% (HR, 0.54; 95% CI, 0.38-0.77). Adjuvant ET reduced the incidence of ipsilateral and contralateral breast cancer (9-year rates; 1.1% vs 6.9%; sHR, 0.17; 95% CI 0.11-0.28, and 1.9% vs 5.2%; sHR, 0.33; 95% CI, 0.22-0.49). Conclusions: Adjuvant endocrine therapy significantly reduces distant metastases, however the absolute difference is small for ER-positive and HER2-negative T1a/bN0M0 breast cancer. Furthermore, it reduced ipsilateral and contralateral breast cancer as well. Shared decision making regarding the omission of adjuvant endocrine therapy is crucial, especially in patients with low risk of distant metastasis.