It was reported previously that serum total sialic acid levels were increased in various cancers including breast cancer. We aimed to evaluate the effect of aromatase inhibitor therapy on serum total sialic acid levels in breast cancer patients. Total sialic acid levels were determined in sera of 20 patients with breast cancer and treated with aromatase inhibitors. Total sialic acid levels were determined by a colorimetric method. Serum total sialic acid levels were 15,06±2,71 mmol/ml and 8,17±5,31 mmol/ml at the beginning and three months after the treatment respectively. Thus, a significant decrease in total sialic acid levels was observed after the treatment (p