We have investigated the possible relation between serum levels of CA 15.3 and disease status in 110 patients after radical mastectomy for breast cancer, with metastatic diffusion. Its persistent elevation was usually related to a very poor prognosis. In patients who died within 18 months the marker was always elevated. In case of progression of the disease, the marker level appeared to be consistently correlated with the general clinical condition. In healthy patients with stable disease the marker remained near the normal range.