Due to the importance of arginase enzyme in different malignant disorders, the purpose ofthe present study was to determine and compare the arginase activity in cancerous cells andtheir normal and benign counterparts. The tissue arginase activity level was evaluated in 30females with breast cancer, in 6 females with benign breast disease and in 9 healthy controlsubjects. The arginase activity levels were significantly increased in malignant breast tissues incomparison to healthy ones, while the difference did not reach the level of significance incomparison to benign breast diseased tissues. Patients with advanced stage showedinsignificantly higher arginase activity compared to those with early stage. In addition,estrogen receptor negative tumors showed insignificant higher arginase activ6ity levelscompared to estrogen receptor positive tumors. Moreover, tissues of premenopausal patientsshowed lower activity levels of arginase compared with those of postmenopausalones.Meanwhile, patients with bad prognosis revealed insignificantly higher activity levels ofarginase compared to those with good prognosis. It could be concluded that tissue arginaseactivity seems to be involved in the biological behaviour of breast cancer and its determinationin cancerous tissues could predict its outcome.