The association of c-erbB-2 gene amplification product (p185) with histologic tumor type in 100 patients with primary breast cancer was determined. In 49 patients with infiltrating ductal carcinoma p185 detection was correlated with histologic findings (tumor grade, lymphnode status, receptor status). Strong positive staining for p185 protein was found in 10 patients (20%) with infiltrating ductal breast carcinoma and correlated with complete negative estrogen/progesterone receptor status and with histologic grade G3. There was neither an association with lymphnode involvement nor was there any to negative estrogen and progesterone receptor status alone. At present, we cannot say whether or not there is a correlation between the degree of c-erbB-2 gene amplification and prognosis. Follow-up studies are necessary to determine whether c-erbB-2 gene amplification allows definition of a specific subset of women who could benefit from adjuvant therapy.