BACKGROUND: Recently, breast cancer incidence and prevalence has been increasing. Patients' health related quality of life is important considerations in the treatment of breast cancer. The EQ-5D-3L is one of most popular instruments to measure health related quality of life. This study was aimed to evaluate the validity and reliability of EQ-5D-3L in post-operative breast cancer patients from Korea. METHODS: A total of 827 patients visiting the ambulatory cancer center of 1 tertiary hospital after breast cancer surgery self-administered the EQ-5D-3L and Functional Assessment of Cancer Therapy-Breast Cancer (FACT-B). We evaluated known-group validity using differences in the EQ-5D-3L index and EQ-VAS score according to demographic and clinical data. The discriminatory ability of the EQ-5D-3L was determined by comparing the mean FACT-B subscale scores between subjects with no problems and subjects with moderate or severe problems in each EQ-5D-3L dimension. Construct validity was evaluated by Pearson correlation coefficients among the EQ-5D-3L index and FACT-B subscales, respectively. Reliability was assessed in terms of test-retest reliability using Cohen's kappa value and intra-class correlation coefficient (ICC). RESULTS: The EQ-5D-3L index and EQ-VAS score were higher in the educated, current radiotherapy and unmarried groups. The correlation of EQ-5D-3L index and subscales for the FACT-B was highest in physical well-being (r = 0.553) and lowest in social well-being (r = 0.199). For reliability, the Kappa values' range was from 0.32 to 0.70, and ICCs of the EQ-5D-3L index and EQ-VAS scores were 0.70 and 0.48, respectively. CONCLUSIONS: This study indicated that the EQ-5D-3L could be a valid health related quality of life instrument for postoperative breast cancer patients.