Extracellular signal-regulated kinase (ERK1/2) is implicated in the malignant behavior of breast cancer cells. However, previous clinical-pathological studies have shown that expression of activated/phosphorylated ERK1/2 is not associated with enhanced proliferation and invasion of mammary carcinomas. ERK1/2 is expressed in the cytoplasm, and activated/phosphorylated ERK1/2 translocates to the nucleus. The aim of this study is to evaluate nuclear phosphorylated ERK1/2 as a biomarker for breast cancer prognosis. The clinical-pathological relation of cytoplasmic/nuclear phosphorylated ERK1/2 was analyzed in 105 surgically resected breast cancer specimens by immunohistochemistry with tissue microarray. The results showed that non-neoplastic breast tissue mainly showed faint phosphorylated ERK1/2 staining. No statistically significant association was found between the level of cytoplasmic phosphorylated ERK1/2 expression and the clinical features of the disease. High nuclear phosphorylated ERK1/2 expression was associated with high grade (poor differentiation, p = = 0.010), high T status (larger tumor size, p = 0.033), and an advanced stage (p = 0.018) of the disease. Thus, nuclear phosphorylated ERK1/2 is associated with enhanced proliferation and invasion of mammary carcinomas and may be a biomarker for breast cancer prognosis and the determination of therapeutic strategies.