To investigate the relationship of breast cancer with serum arginase activity and nitric oxide levels. Arginase (L-arginine ureohidrolase, EC. 3.5.3.1) is the last enzyme of urea cycle which converts arginine into urea and ornithine. nitric oxide, a product of L-arginine and nitric oxide synthetase reaction, is a hormone, a reactive oxygen species, neurotransmitter, mediator, cytoprotective molecule, and the only endogenous molecule that acts as a cytotoxic molecule. This study was done at Gaziantep University Research Hospital with 30 breast cancer patients (30-77) and with 34 healthy people (30-75) to diagnose breast cancer. The majority (n=28) was ductal while the rest were medullary and papillary cancers. Serum arginase activity was measured by thiosemicarbazide diasetilmonoksim urea method in U/L that was modified. Also, serum nitric oxide levels were measured by the Griess method in terms of mmol/L. Serum arginase activity was determined as 17.8±2.5 (X±SE) U/L in the patient group and 6.8±0.9 U/L in the control group; the cancer patients showed a significant increase (t=3,649, p <0.01). Serum nitric oxide levels were found 139.4 ± 7.4 mmol / L in the breast cancer group, and 95.9 ± 7.6.1 mmol/L in the healthy group. nitric oxide levels were found to be significant in the analysis (t=4,197, p<0.001). In this study, a significant increase in serum arginase and nitric oxide activity was observed for the breast cancer patients, and it was concluded that both could be important for the diagnosis of breast cancer and for its treatment.