CA 27.29 is a tumor marker useful in the management of patients with metastatic carcinoma of the breast. It is used to monitor the course of breast cancer, patient response to treatment, and disease recurrence. Elevated serum CA 27.29 concentrations are found in 5% of stage I, 29% of stage II, 32% of stage III and 95% of stage IV carcinoma of the breast. Most (96%) patients with a CA 27.29 increase of greater than 25% have disease progression. Most (nearly 100%) patients with a CA 27.29 decrease of greater than 50% are responding to treatment.