Carbohydrate antigen 15-3(CA15-3) is a tumor marker used to monitor certain cancers, especially breast cancer. Elevated CA15-3, in conjunction with alkaline phosphatase, was shown to increase chances of early recurrence in breast cancer. CA15-3 has been suggested as a marker of distant metastasis (M+ disease) in breast cancer. CA15-3 might be assayed alongside a bone scan to confirm positive or negative results. Another role might be as a screen for breast cancer metastases in departments with limited access to bone scans and other imaging facilities. CA15-3 might also be used in monitoring patients for the development of distant metastases during follow-up. It is, however, unlikely that CA15-3 can substitute directly for a bone scan or other imaging currently used routinely by a department. Clinical trials are now necessary to determine the effect of using tumour markers such as CA15-3 on patient morbidity and mortality.