Locally advanced breast cancer (LABC) continues to be a common presentation of breast cancer worldwide. Various serum markers have been studied to know the behaviour, metastasis and prognosis of breast cancer without going through any invasive procedure. CA 15-3 measures the protein product of the MUC 1 gene and is the most commonly used serum marker in breast cancer. The main use of serum CA 15-3 is monitoring therapy response and elevated levels are associated with metastatic disease and in non-metastatic patients, it is associated with poor prognosis. The aim of the study was to know the role of serum CA 15-3 level in diagnosis, prognosis and therapy response in patients with locally advanced breast cancer and to assess its relationship with other variables like tumor size, microscopic grade, nodal burden and Ki-67 antigen expression. Serum CA 15-3 levels significantly decreased after neo-adjuvant chemotherapy and mastectomy and it remained elevated in patients with suspected or hidden metastasis. Serum CA 15-3 levels exhibited positive statistically significant correlation with tumor size. DOI: 10.21276/AWCH.1782