HER2 is an important predictive marker for response to trastuzumab and lapatinib in breast cancer. It is also a powerful prognostic marker in node-positive patients. Although standardized assays are used to help select patients for anti-HER2 therapy, there are no standardized criteria for assessing HER2 as a prognostic marker. Recent data using quantitative image analysis suggest that both high and low HER2 expression are associated with poor clinical outcome. Using the immunohistochemical scoring criteria currently recommended by the College of American Pathologists and American Society of Clinical Oncology to help select patients for trastuzumab, we evaluated HER2 protein expression in tumor tissue microarrays of 91 node-positive patients with invasive breast carcinoma treated with mastectomy and doxorubicin-based chemotherapy without trastuzumab and without irradiation with a median follow-up of 12.5 years. A wide range of HER2 expression (HER2 >or=1+) in the primary tumor was significantly associated with decreased locoregional recurrence-free survival (P=0.014), decreased disease-specific survival (P=0.001), and decreased overall survival (P=0.001). Even in the subset considered HER2 negative by current College of American Pathologists and American Society of Clinical Oncology guidelines, HER2=1+ was associated with worse outcome than HER2=0 in this patient cohort. The association between HER2 >or=1+ and worse outcome had the greatest statistical significance in the hormone receptor-positive subset of patients. These findings support the hypothesis that low-level HER2 expression may have significant clinical implications. Although the assessment of HER2 expression is most important for predicting response to anti-HER2 therapy, detection of low-level HER2 expression might also be useful in helping to select a more aggressive treatment regimen for patients ineligible for anti-HER2 therapy.