The significance of c-erbB-2 protein expression was immunohistochemically investigated in 32 patients with stage III breast cancer with regard to area, shape factor and DNA content of cancer cell nuclei. Thirty-one percent of the tumors showed c-erbB-2-positive cell membrane staining (10 out of 32 cases), and the cancer cells of the c-erbB-2-positive tumors had larger and more irregular nuclei with an increased DNA content. All patients with c-erbB-2 protein-positive tumors died of the disease within six years of their mastectomy. It can be concluded that positive immunohistochemical staining of c-erbB-2 protein indicates an aggressive biological behavior of the cancer cells as well as a poor prognosis in patients with stage III breast cancer.