Many serological markers have been utilized to indicate the status, risk, or presence of breast cancer. In May 1996, the American Society of Clinical Oncology (ASCO) convened a Tumor Marker Panel and determined clinical practice guidelines for the use of tumor markers in breast cancer. Eight markers containing carcinoembryonic antigen (CEA) and CA15-3 were evaluated and assigned by expert reviewers to be valuable markers of breast cancer. CA15-3 recognizes a mucin-like glycoprotein, MUC-1, which is frequently expressed in breast cancer tissues. BCA225, which may recognize antigens similar to MUC-1 glycoprotein, are sensitive and specific markers for breast cancer. However, it is not recommended to measure the 2 markers in combination. The measurement of carboxy-terminal telopeptide of type I collagen (I CTP) is worthwhile as a serological diagnostic method of bone metastasis from breast cancer. Other markers such as erbB-2, CYFRA 21-1 and PTHrP are candidates for clinical utilization as tumor markers in breast cancer.