The American Joint Committee on Cancer (AJCC) anatomic stage groups have arguably been the most powerful in predicting breast cancer outcomes. In the present study, we validated the newly established AJCC clinical prognostic stage groups and compared them with the anatomic stage groups in a cohort of 3322 breast cancer patients. We found that the application of the clinical prognostic stage assigned 52.4% of cases to a higher or lower stage compared with the anatomic stage. The Cox proportional hazard model demonstrated superior discriminatory power for the clinical prognostic stage, especially for patients with clinical prognostic stage I and III disease. Background: The American Joint Committee on Cancer anatomic stage/prognostic group template arguably is the most powerful in predicting breast cancer outcomes because it considers the primary tumor, regional lymph node involvement, and presence of distant metastasis. However, other tumor and host characteristics have also been proved to be of prognostic value, including histologic grade and estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 status. Thus, the 8th edition of the American Joint Committee on Cancer consolidated these factors into clinical prognostic stage groups. Materials and Methods: We validated the clinical prognostic stage groups compared with the anatomic stage groups in a cohort of 3322 breast cancer patients. Results: Compared with the anatomic stage, application of the clinical prognostic stage assigned 27.7% and 24.7% of cases to higher and lower stage groups, respectively. In 14% and 2.8% of cases, the assignment of clinical prognostic stage varied by 2 and 3 anatomic stages up or down, respectively. The Cox proportional hazard model demonstrated superior discriminatory power for clinical prognostic stage (overall χ 2 , 464.5; P <.0001 vs. χ 2 , 363.9; P <.0001). A pairwise comparison revealed that significant improvement was especially observed for patients with clinically prognostic stage I and III disease compared with that of the anatomic stage groups. Conclusion: The new clinical prognostic stage provides a more powerful, yet imperfect, tool for predicting breast cancer outcomes. Further refinement of this system might be necessary in the pursuit of precision medicine.