In order to assess the utility of the tumor-associated antigen CA15-3 in the diagnosis of breast cancer, this new tumor marker was measured pre-operatively in 1342 patients. This group comprised 509 patients with malignant disease (134 with breast cancer and 375 with other malignancies not involving the breast) and 833 patients with benign surgical diseases (95 patients with fibroadenoma of the breast, 738 with other benign diseases). The results were compared with those for carcino-embryonic antigen (CEA) in the diagnosis of breast cancer. CA15-3 was above the normal limits of 25 U/ml in 31% of the patients with breast cancer, in 22% of patients with other malignancies, and in 9% of patients with benign diseases. CEA was elevated in 26% of patients with breast cancer (greater than 3 ng/ml). CA15-3 levels were above 50 U/ml in 13% of the breast cancer patients, in 6% of patients with other malignancies, and in 0.2% of the patients with benign diseases. There was a good correlation between CA15-3 level and tumor stage in breast cancer. CA15-3 serum levels were over 50 U/ml in respectively 0%, 2%, 13%, and 73% of the patients with stages I, II, III, and IV. CA15-3 and CEA were also determined in 671 patients who had received initial curative surgery of breast cancer, and who regularly attended our follow-up clinic. CA15-3 was found to be more sensitive than CEA in detecting recurrences of breast cancer.(ABSTRACT TRUNCATED AT 250 WORDS)