Amplification of the Neu oncogene (c-erbB-2) has been reported by various researchers as a marker for poor clinical outcome in patients with breast cancer. We have performed immunohistochemical staining using a polyclonal antibody to the Neu oncoprotein on formalin-fixed material from normal breast, benign breast lesions, and 102 stage I node-negative breast cancers. Hybridization studies were also performed on 66 of the breast cancer cases. In the cancers 33% of cases showed positive staining of the in situ and invasive component, whereas only 25% of cases showed amplification of the Neu oncogene. The staining pattern in the tumor cells was cytoplasmic with plasma membrane accentuation. Focal positive cytoplasmic staining was noted in some cases of fibrocystic disease, fibroadenoma, and normal breast duct epithelium. Myoepithelial cells and smooth muscle of blood vessels also showed a positive reaction. This study shows that the Neu oncoprotein can be demonstrated on formalin-fixed material from normal, benign, and malignant breast lesions. In the breast cancers the differences in the number of cases showing amplification and those showing a positive immunohistochemical reaction could be due to increased transcriptional activity. It is possible that the node-negative patients whose tumors express the Neu oncogene may correspond to the group of patients who are expected to have a poor prognosis.