Patients with lymph node-negative breast carcinoma (LNNBC) and positive hormone receptor (HR) status during estrogen-based endocrine therapy have different prognoses. The contribution of HER-2 (human epidermal growth factor receptor-2) has not been extensively evaluated. We stained 230 LNNBCs for estrogen and progesterone receptors (ER and PR) and HER-2. HER-2 gene status was studied in 150 randomly selected tumors by chromogenic in situ hybridization and cases with discordant or nondefinitive results by fluorescence in situ hybridization. Patients with ER+ and/or PR+ tumors were treated with tamoxifen. We found positive expression of ER, PR, and HER-2 in 73.7%, 67%, and 27.8%, respectively, and HER-2 amplification in 18.0%. Poorer outcome was seen for patients with ER+ and/or PR+/HER-2 overexpressing tumors and as a trend for patients with HER-2 amplification. ER/PR and HER-2 expression showed an independent prognostic value. In LNNBCs with positive HR status, HER-2 overexpression and/or amplification confer an aggressive tumor phenotype, and this might be related to tamoxifen unresponsiveness.