Adenomyoepithelioma, an uncommon benign breast tumor in which the proliferation of myoepithelial cells is a conspicuous component, was examined in 18 women. The initial clinical finding was usually a mass in the periphery of the breast. Grossly, adenomyoepithelioma produced a circumscribed, firm tumor (average, 1.5 cm) with nodularity and cysts noted in some instances. Proliferating myoepithelial cells and gland-forming epithelial cells were conspicuous in all cases. In most, myoepithelial cells were polygonal and had clear vacuolated cytoplasm. Myoid, spindle-cell differentiation was rarely prominent, but cells of this type were present focally in most cases. Intraductal papillary adenomyoepitheliomatous hyperplasia was present within the tumors and sometimes in surrounding ducts. Immunohistochemical studies distinguished epithelial from myoepithelial cells. Hormone receptor studies showed four tumors were estrogen receptor positive, two were estrogen receptor negative, and five were progesterone receptor negative. One patient who underwent immediate mastectomy is well 4 years later. Two of the 17 women treated by local excision had recurrent adenomyoepithelioma in the same breast. All 18 patients remain well from 0.5 to 5.5 years after diagnosis. No patient had prior, coexistent, or subsequent adenocarcinoma of either breast. These findings indicate that adenomyoepithelioma is a benign tumor that can be adequately treated by complete local excision.