Aromatase is the key enzyme in intratumoral estrogen production in postmenopausal breast cancer and third generation aromatase inhibitors suppress this enzymatic reaction effectively. Aromatase inhibitor is administered to metastatic breast cancer patients customarily in which estrogen receptor had been demonstrated only in the primary tumor, not the metastatic sites. The status of aromatase in metastatic sites has not been well-characterized to date. We immunolocalized aromatase in 46 estrogen receptor positive primary breast cancers and paired metastatic lymph nodes, using immunohistochemistry. Immunoreactivity was detected in 44/46 primary tumors and 40/46 metastatic lymph nodes. A significant correlation was detected between the status of aromatase in primary and metastatic sites. Aromatase immunoreactivity was correlated with age, size of primary tumor and Ki-67 index. Aromatase immunoreactivity was also detected in adipose tissue surrounding the lymph nodes. In conclusion, aromatase status in primary tumors generally represents its status in metastatic lymph nodes. This indicates that the endocrine environment of estrogen receptor positive tumors remain stable during the metastatic process.