The diagnostic value of a new tumor marker, c-erbB-2, was studied in the sera of 50 healthy subjects, 58 patients with benign breast diseases, and 413 patients with breast cancer (186 locoregional, 185 with advanced disease, and 42 with no evidence of disease). Using 15 U/ml as the cut-off, no healthy subjects or patients with benign diseases and only 2.4% of no evidence of disease patients had elevated serum levels. Abnormal c-erbB-2 levels were found in 29% (101/370) of the patients with breast carcinoma (locoregional 9%, metastases 45.4%). CEA (cut-off 5 U/ml) and CA 15.3 (cut-off 35 U/ml) sensitivity was 18% and 16% in patients with locoregional disease and 61% and 70% in those patients with advanced disease, respectively. A trend toward higher serum levels of all three tumor markers in patients with nodal involvement or greater tumor size was found, but was statistically significant only with CEA (p < 0.01). By contrast, c-erbB-2 was related to steroid receptors, in both locoregional and metastatic tumors. When the prognostic value of these markers was evaluated, patients with abnormally high presurgical CEA and c-erbB-2 had a worse prognosis than those patients with normal values, in both node-negative (p < 0.05 and p < 0.001, respectively) and node-positive patients (p < 0.556 and p < 0.001, respectively). By contrast, no relationship was found between CA 15.3 values and prognosis. Multivariate analysis showed that CEA and c-erbB-2 were also prognostic factors. The correlation between serum and tissue levels of c-erbB-2 was studied in the tumors of 161 patients. Significantly higher c-erbB-2 serum levels were found in patients with overexpression in tissue by immunohistochemistry, in both locoregional and advanced disease (p = 0.0001). Serum concentrations in patients with advanced disease were related to the site of recurrence, with significantly higher values in patients with metastases (mainly in those with liver metastases) than in those with locoregional recurrence. In summary, c-erbB-2 serum levels seem to be a useful tumor marker in the prognosis of patients with breast cancer. Using all three tumor markers, sensitivity was 35% in patients with locoregional breast cancer and 88% in patients with recurrence.