Background/Aim: This study evaluated the prognostic value of the 8th edition of American Joint Committee on Cancer (AJCC) cancer staging system for patients with internal mammary lymph node (IMN) metastases. Materials and Methods: Of the patients with breast cancer who were treated between 2009 and 2013, 66 were diagnosed as cN3b. We restaged the patients and analyzed the prognostic value of the prognostically staged groups. Results: With a median follow-up of 53.9 months, the 5-year overall survival rates of patients with IIIA, IIIB, and IIIC stages were 100%, 95%, and 50% (p=0.001), while the progression-free survival rates were 100%, 83%, and 50% (p=0.005). Conclusion: Despite the small number of patients, the prognostic stage provided accurate information for IMN metastasized breast cancer, which will lead to more accurate prognosis predictions and optimal treatment selection.