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To establish a risk scoring system for predicting locoregional recurrence (LRR) and explore the potential value of radiotherapy in T1 to T2 node-negative breast cancer patients treated with mastectomy. From January 2001 to February 2008, a total of 353 node-negative T1 to T2 breast cancer cases treated with mastectomy without adjuvant radiotherapy were retrospectively analyzed. Preliminary screening of the prognostic factors was accomplished by Kaplan–Meier univariate analysis, and survival curves between different groups were compared by log-rank test. Risk factors were determined using Cox proportional hazards model. A categorical risk scoring system was generated according to the Cox model, weighing the relative importance of each risk variable. Median follow-up was 115.7 months (range, 1.2–238.4 months). The overall 5-year locoregional recurrence-free survival (LRFS) was 89.8% (95% confidence interval [CI] = 86.7%–92.9%). Chest wall (53.8%) was found to be the most common site of LRR, followed by supraclavicular nodes (48.7%). Age ≤40 years, primary tumor size ≥4.5 cm and number of nodes resected ≤10 were found to be independent factors for poor prognosis of LRR. Two risk stratifications based on the scoring system were subsequently obtained. The 5-year LRFS was 91.6% (95% CI = 88.5%–94.7%) with low risk (score <2) and 75.7% (95% CI = 61.8%–89.6%) with high risk (score ≥2), respectively (χ2 = 7.544, P = .006). In addition, significant differences in overall survival (P = .045) and disease-free survival (P = .019) were presented between them. Patients with T1-2N0M0 breast cancer achieved favorable prognosis in general. Those with risk factors, including age ≤40 years, primary tumor size ≥4.5 cm and number of nodes resected ≤10, were at higher risk of LRR. The established scoring system could help to distinguish the subgroups that might potentially benefit from postoperative radiotherapy.