A new antigen associated with breast cancer, CA 15.3, was determined in the serum of 690 breast cancer patients and controls. After establishing a maximum normal level of 40 U/ml in 140 healthy subjects and 350 patients with benign diseases, CA 15.3 was investigated in 190 breast cancer patients: CA 15.3 serum levels were statistically different in patients with no evidence of disease (20.6 +/- 11.2 U/ml), metastatic patients in response (33.5 +/- 24.0 U/ml) and metastatic patients not responding to therapy (stable disease, 98.9 +/- 50.4; progression, greater than 200). CA 15.3 serum levels seem to correlate with the extent of metastatic breast cancer. Further studies are needed to establish the role of this marker in the management of breast cancer patients.