The HER-2/neu proto-oncogene (also known as c-erb B-2) is homologous with, but distinct from, the epidermal growth factor receptor. Amplification of this gene in node-positive breast cancers has been shown to correlate with both earlier relapse and shorter overall survival. In node-negative breast cancer patients, the subgroup for which accurate prognostic data could make a significant contribution to treatment decisions, the prognostic utility of HER-2/neu amplification and/or overexpression has been controversial. The purpose of this report is to address the issues surrounding this controversy and to evaluate the prognostic utility of overexpression in a carefully followed group of patients using appropriately characterized reagents and methods. In this report we present data from a study of HER-2/neu expression designed specifically to test whether or not overexpression is associated with an increased risk of recurrence in node-negative breast cancers. From a cohort of 704 women with node-negative breast cancer who experienced recurrent disease (relapsed cases) 105 were matched with 105 women with no recurrence (disease-free controls) after the equivalent follow-up period. Immunohistochemistry was used to assess HER-2/neu expression in archival tissue blocks from both relapsed cases and their matched disease-free controls. Importantly, a series of molecularly characterized breast cancer specimens were used to confirm that the antibody used was of sufficient sensitivity and specificity to identify those cancers overexpressing the HER-2/neu protein in this formalin-fixed, paraffin-embedded tissue cohort. In addition, a quantitative approach was developed to more accurately assess the amount of HER-2/neu protein identified by immunostaining tumor tissue. This was done using a purified HER-2/neu protein synthesized in a bacterial expression vector and protein lysates derived from a series of cell lines, engineered to express a defined range of HER-2/neu oncoprotein levels. By using cells with defined expression levels as calibration material, computerized image analysis of immunohistochemical staining could be used to determine the amount of oncoprotein product in these cell lines as well as in human breast cancer specimens. Quantitation of the amount of HER-2/neu protein product determined by computerized image analysis of immunohistochemical assays correlated very closely with quantitative analysis of a series of molecularly characterized breast cancer cell lines and breast cancer tissue specimens.(ABSTRACT TRUNCATED AT 400 WORDS)