PURPOSE: Preclinical studies have shown that human epidermal growth factor receptor 2 (HER2) status is associated with resistance to radiotherapy (RT). In this study, we evaluated the overall survival of a T1N0M0 breast cancer cohort in Korea according to the use of RT and the HER2 status. METHODS: We analyzed data collected from 11,552 patients with invasive breast cancer who were enrolled in the Korean Breast Cancer Society Registration Program between 1999 and 2007. Data on the TNM stage, estrogen receptor status, progesterone receptor status, HER2 status, operation method, and the use of RT were analyzed. RESULTS: The median follow-up period was 51 months. A significant improvement in overall survival after RT was observed only in the HER2(-) group. In this group, the 10-year overall survival rate was 95.5% for patients who did not receive RT and 96.3% for patients who received RT (p=0.037). In contrast, in the HER2(+) group, RT was not associated with a survival benefit (p=0.887). Multivariate analysis showed that RT was significantly associated with a reduction in mortality in the HER2(-) group (hazard ratio, 0.738; 95% confidence interval, 0.549-0.993; p=0.045). CONCLUSION: We found that postoperative RT was not associated with a survival benefit in HER2(+) breast cancer patients, suggesting that HER2(+) breast cancers could be RT resistant.