Introduction: Breast cancer (BC) is most common cancer site in women worldwide. The CA 15-3 is tumor associated antigen which detects soluble forms of MUC-1 protein. In normal breast tissue MUC-1 is expressed in the duct and acini, but with neoplastic transformation normal cell polarization and tissue architecture is disrupted leading to shedding of MUC-1 in the blood, where it can be measured by immunoassay. It is the most widely used serum marker in patients with BC. Its main use is for monitoring therapy in patients with metastatic disease.

Aim of the work: Aim of this work was to detect the diagnostic value of CA 15-3 in female patients with BC.

Patients and Methods: This study was carried on one hundred women, included fifty female patients with BC presented to Medical Oncology Department at Sohag University Hospital from 2015 to 2017, their age ranged from (25-70 years). Patients were diagnosed by clinical examination and confirmed by mammography and surgical biopsies; fine needle aspiration cytology (FNAC) or core needle biopsy (CNB) and CA15-3. Other fifty women were 25 healthy control not relatives to the patients and the other 25 were first degree relatives to the patients.

Results: Our study suggested that CA 15-3 is abnormal in the majority of patients with breast cancer.

Conclusion: Serum markers in BC are helpful for clinicians in providing more effective management of the disease. To this end, different markers have been proposed in the last years. In particular, MUC1 (CA 15-3) is the most widely used and investigated in the breast cancer follow-up period

Key words: CA 15-3, Breast Cancer.