CA15-3 (Cancer Antigen 15-3) is a tumor marker used to monitor certain cancers, especially breast cancer. It is found on the surface of many types of cancer cells and shed into the blood stream. It is used to monitor advanced, i.e. metastatic, cancer. Elevated CA15-3, in conjunction with alkaline phosphatase (ALP), was found to be associated with an increased chance of early recurrence in breast cancer.

CA15-3 and associated CA27.29 (aka BR27.29) are different epitopes on the same protein antigen product of the breast cancer-associated MUC1 gene. CA27.29 has enhanced sensitivity and specificity and has therefore surpassed CA15-3 as a serum tumor marker. CA27.29 is elevated in 30% of patients with low-stage disease and 60 to 70% of patients with advanced-stage breast cancer.

Both CA27.29 and CA15-3 may be elevated in patients with benign ovarian cysts, benign breast disease, and benign liver disease. Elevations may also be seen in cirrhosis, sarcoidosis and lupus.