The purpose of this study is to address the hypothesis that activatedmitogen-activated protein kinase (MAPK; extracellular signal-regulated kinases 1 and 2) has a role in breast tumorigenesis, breast cancer progression, and the development of tamoxifen resistance. H-score analysis and a specific antibody for the immunohistochemical detection of activated MAPK in formalin-fixed, paraffin-embedded tissue sections were used to compare expression in: (a) human breast tumors and their matched adjacent normal breast tissue; (b) primary human breast tumors and their matched lymph node metastases; and (c) primary breast tumors from patients who later proved to be sensitive or resistant to tamoxifen treatment. Active MAPK expression was detected in 48% of primary human breast tumors and was significantly increased in tumors compared with adjacent normal breast (Wilcoxon test, P = 0.027). A significant positive association (chi(2), P = 0.02; n = 55) was obtained between active MAPK and the presence of lymph node metastases. Moreover, increased active MAPK (Wilcoxon test, P = 0.0098) was found in concurrent lymph node metastases compared with primary breast tumors. No significant difference in active MAPK was found in primary tumors of patients who later responded to tamoxifen or did not respond to tamoxifen. These data suggest that active MAPK is a marker of breast cancer metastasis and has a role in the metastatic process. However, active MAPK is unlikely to be a marker of endocrine sensitivity or involved in de novo tamoxifen resistance.