The relationship between c-erbB-2 serum positivity and prognosis was evaluated in 80 patients with metastatic breast cancer. Using 120 fmol/ml as a cutoff level, elevated concentrations were found in 31 patients (38.8%) at the time of detection of metastases. Menopausal status, steroid receptor status, site of recurrence, initial tumor size, initial degree of nodal involvement as well as relapse-free interval were unrelated to c-erbB-2 serum positivity. In addition, no association could be found between adjuvant chemotherapy and positive c-erbB-2 concentrations. Patients with elevated c-erbB-2 levels showed a lower response rate (including complete remission, partial remission, no change) to first-line therapy than those with normal levels (29 vs. 59%, p < 0.01). The median survival time after relapse was 12 months (CI: 3-22 months) for the c-erbB-2-negative patients and only 6 months (CI: 3-8 months) for the c-erbB-2-positive group (p < 0.01). In the multivariate analysis, while c-erbB-2 levels at the time of primary surgery had no significant impact on survival in metastatic breast cancer, serum c-erbB-2 turned out to be the strongest factor for predicting survival after relapse.