Breast cancer is both one of the most common and one of the most treatable of all human malignancies. It has been suggested by various investigators that sialic acid increases in the sera of cancerous patients. In cancer patients, an increase in the levels of serum sialic acid may also be due to an increase in the activity of serum or tissue sialidase, The purpose of the present investigation was to determine whether the concentration of sialidase in serum and breast tissue could be used as a tumor marker in breast cancer. In this study; serum sialidase levels in 26 patient with breast cancer and 31 controls were found to be 77.04 +/- 25.07 U/l and 55.56 +/- 7.50 U/l, respectively. The mean tissue sialidase levels in 26 breast cancer patients and 13 controls were 39.76 +/- 17.03 U/g protein acid 14.30 +/- 7.09 U/g protein, respectively. Serum and tissue sialidase levels in breast cancer were significantly higher than those found in the control group (P < 0.001). The mean serum and tissue sialidase levels in 14 Grade I-II and 12 Grade III breast cancer patients were found to be 67.73 +/- 11.87 U/l and 33.41 +/- 12.17 U/g protein and 87.89 +/- 31.94 U/l and 47.17 +/- 19.30 U/g protein, respectively. Also we found a significant difference between the levels of serum and tissue sialidase in Grade I-II and III (P < 0.05), (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.