Breast cancer is the most common malignant tumor of women around the world. Prostate cancer contributes significantly to the overall cancer burden, being the most frequent malignant neoplasia in men. Therefor1e, great medical and scientific efforts are continually invested into understanding the disease's pathology and finding new methods for its diagnosis and treatment. It was interesting to investigate Arginase activity in blood serum during women breast and men prostate cancer. In this study, a significant increase in blood serum arginase activity could be important for the early diagnosis of mentioned cancers and for their treatment. Taking into consideration that through inhibition of Arginase activity polyamine quantity is changed, which can have influence on the cancer cell metabolism, it can find practical application in medical practice.