Background: This study investigated the survival benefit of radiotherapy (RT) of the supra- and infraclavicularlymphatic drainage area in Chinese women with T1-2N1M0 breast cancer receiving mastectomy. Methods: A totalof 593 cases were retrospectively reviewed from 1998 to 2007. The relationship between supra- or infraclavicularfossa relapse (SCFR) and post-operative RT at the supra-/infraclavicular lymphatic drainage area was evaluated. Results: The majority of patients (532/593; 89. 8%) received no RT while 61 patients received RT. The medianfollow-up was 85 months. Among patients without RT, 54 (10. 2%) developed recurrence in the chest wallor ipsilateral SCFR. However, none of the 61 patients who underwent RT demonstrated SCFR. One patientwho received RT (1. 6%) experienced recurrence in the chest wall. Univariate analysis revealed that age andmolecular subtype (both P < 0. 05) were two prognostic factors related to supraclavicular and infraclavicularfossa relapse-free survival (SFRFS). Multivariate analysis revealed that only Her-2 positive status (P = 0. 011) wasan independent predictor of SFRFS. RT had no influence on distant metastasis (P = 0. 328) or overall survival (P= 0. 541). SCFR significantly affected probability of distant metastasis (P < 0. 001) and overall survival (P < 0.001). Conclusion: Although RT was not significantly associated with SFRFS, postoperative RT was significantlyassociated with a lower locoregional (i. e. , supraclavicular/infraclavicular and chest wall) recurrence rate. SCFRsignificantly influenced distant metastasis-free survival, which significantly influenced the overall survival of T1-2N1M0 breast cancer patients after mastectomy. Thus, prophylactic RT is recommended in T1-2N1M0 breastcancer patients, especially those who have Her-2 positive lesions.