A clinico-pathologic study with an immunohistochemical examination for c-erbB-2 expression in 54 cases of ductal carcinoma in situ and 16 cases of Paget's disease of the breast was performed. c-erbB-2 oncoprotein overexpression was observed in 45% (24/54) and 88% (14/16) of ductal carcinoma in situ and Paget's disease, respectively. The overexpression of c-erbB-2 oncoprotein was significantly correlated with the nuclear grade of tumors and inversely with the status of the estrogen receptor. c-erbB-2 was positive in 4 out of 5 patients with metastasis to axillary lymph nodes and 3 out of 4 patients who died of the disease. Prognostic significance of c-erbB-2 oncoprotein in ductal carcinoma in situ was highly suggested. The expression of c-erbB-2 oncoprotein in Paget's disease was well correlated with coexisting infiltrating or in situ ductal carcinoma. The high positive rate of c-erbB-2 oncoprotein in ductal carcinoma with Paget's disease could be understood with a recent hypothesis that c-erbB-2 oncoprotein is involved in promotion of cell motility and the spread of carcinoma cells.