The immunohistochemical expression and distribution of epidermal growth factor receptor (EGFr) in mammary myoepithelial cells (MECs) in normal tissue, benign epithelial proliferative lesions, and in situ carcinoma was performed. Results of the current study demonstrated that MECs stained constantly and strongly for EGFr, creating an outer continuous ring surrounding the epithelium of ducts and acini in healthy, in proliferative epithelial lesions, and in in situ carcinoma, both of ductal and lobular type. Foci of microinvasion were easily appreciated for the complete loss of EGFr immunostaining. Epidermal growth factor receptor expression in normal epithelia ranged from negative to weakly positive; it was positive in hyperplasia, whereas it was not constantly negative in in situ carcinoma. In conclusion, immunohistochemical staining for EGFr is diagnostically useful for MEC identification. The specific EGFr in MECs leads the authors to suggest that its expression may be related to the recently recognized high-specialized paracrine function by which the MECs exert the natural mechanical and functional role in the juxtaposition between epithelium and stoma.