Aim: Two oestrogen receptors (ERs) have been identified to date-the "classic" ERα and the more recently described ERβ. Although much is known about ERα at the mRNA and protein levels, our knowledge of the expression and distribution of ERβ protein is much more limited. The aim of this study was to compare the cellular distribution of ERα and ERβ in normal human mammary gland. Methods: Formalin fixed, paraffin wax embedded material was obtained from reduction mammoplasty specimens, normal tissue adjacent to breast tumour, or fibroadenoma. Sections were immunohistochemically stained for ERα, ERβ, and the progesterone receptor. The staining pattern for each antibody was evaluated and compared. Results: ERα was restricted to the cell nuclei of epithelial cells lining ducts and lobules. Although ERβ was also seen in these cells, additional strong staining was detected specifically in the cell nuclei of myoepithelial cells. Occasional staining was seen in surrounding stromal and endothelial cell nuclei and in lymphocytes. Conclusions: ER subtypes have distinct distribution patterns in the normal mammary gland. The widespread distribution of ERβ suggests that it may be the dominant ER in the mammary gland where it may be acting as a natural suppressor.