Breast cancer is the most prevalent cancer and the second cause of cancer death after lungrncancer in women. The combination of its high incidence and relatively good prognosisrnmake breast cancer the most prevalent cancer in the world today. Worldwide, it is estimatedrnthat more than one million women are diagnosed with breast cancer every year, and morernthan 410,000 will die from the disease, representing 14% of female cancer deaths.rnMoreover, breast cancer incidence rates have been reported to be increasing by up to 5%rnper year in many populations in developing countries.rnThe pattern of enzymatic alterations may have an intimate correlation with the malignantrnstate and the progression of cancerous cells in the tumor. Cancer cells may differ from theirrnnormal counterparts in the activities or concentration of certain enzymes, includingrnarginase. It is hypothesized that rapid tumor cell proliferation and immune suppressionrnassociated with tumors is partly attributed to the enzyme arginase. So, the present studyrnwas aimed at determining arginase activity in blood and biopsy sample of breast cancerrnpatients referred to St Paulâ€™s hospital Millennium Medical College, Zewditu MemorialrnHospital, Minilik Hospital and National Hospital from April to October 2014.rnA comparative cross sectional study was conducted and convenient sampling techniquernwas employed to include all patients who were presented with breast CA and underwentrnmajor breast surgery within the study period.rnThe result indicated the median preoperative arginase activity in plasma and PBMC fromrnwomen with breast cancer was 72.62 and 729.1 mU/mg protein respectively. It has alsornshown that the pattern of arginase activity in breast cancer patients was significantlyrndifferent from apparently healthy people. Elevated enzyme activity was also demonstratedrnin tumor microenvironment with median activity of 3119mU/mg. The enzyme level wasrnalso found to be correlated with tumor stage. The more advanced the breast cancer, thernmore aggressive the tumor is and the higher the serum and PBMC level of arginase enzymernactivity suggesting its role in tumor progression. In conclusion, Arginase might be thernpotential candidate to be used as a biological marker for breast cancer diagnosis, prognosisrnand disease progression combined with other parameters. It might also be the possiblerntarget for future therapeutic approaches intended to treat cancers tough intensive researchrnis required.