Breast cancer is the most common type of cancer among women worldwide. This study investigated the expression and clinical significance of activating transcription factor 3 (ATF3) in human breast cancer and its relationship with the clinical outcome of breast cancer. Materials and Methods : ATF3 expressions were detected in 114 primary breast cancer tissues and 114 adjacent normal tissues using immunohistochemistry (IHC) assay. Categorical variables were statistically compared by chi-square or Fisher's exact test. Survival curves were evaluated using the Kaplan-Meier method and comparisons of survival rates were tested using a Log-rank test. Results : IHC analysis showed that the positive expression of ATF3 protein was detected in breast cancer tissue with a positive ratio of 76.3%, and the positive ATF3 expression in adjacent normal breast tissue was 13.2%, which is lower than that in breast cancer tissue samples (P<0.01). Furthermore, ATF3 expression showed significant correlation with TNM stage, invasion, lymph node metastasis and number of metastatic lymph nodes (P=0.038, P=0.029, P=0.026, and P=0.039 respectively), and did not correlate with patients' age and tumor size (P>0.05). A significant difference in overall survival rate was found between the patients with positive expression of ATF3 protein and those with negative expression (P=0.041). Conclusion : Increased ATF3 expression participate in the tumorigenesis, invasion and metastasis of breast cancer, and ATF3 may be useful as a new prognostic indicator for breast cancer patients.