The estrogen receptor (ER) is the most useful marker currently available for breast cancer, being used both to predict response to therapy and assess prognosis. Recently, a new form of the ER, known as ER-beta, was identified. In this preliminary study we show that ER-beta mRNA was expressed less frequently in breast cancers than ER-alpha. ER-alpha but not ER-beta levels correlated with ER protein as determined by ELISA. We conclude that ER-beta is expressed in approximately 50% of breast cancers but it does not appear to be detected by a widely available ELISA.