The clinical, histologic, cytologic, and ultrastructural features of three adenoid cystic carcinomas of the breast are presented. All three neoplasms occurred in postmenopausal women. One patient was treated by wide re-excision of the biopsy site. Two were treated by modified radical mastectomy; no axillary lymph node metastases were found. None of the neoplasms has recurred. Neither estrogen nor progesterone receptors were detected in any of the neoplasms. The histopathologic criteria for the diagnosis of adenoid cystic carcinoma are described, and the differential diagnosis is discussed. Since the cytopathologic features of adenoid cystic carcinoma of the breast appear to be distinctive, the neoplasm can be diagnosed by fine-needle aspiration based upon the presence of the following features: 1) a cellular smear; 2) a uniform population of small, basaloid tumor cells; and 3) small globules of mucoid material surrounded by a rim of neoplastic cells. Ultrastructurally, the predominant cell type has the appearance of a modified myoepithelial cell, but true lumina lined by microvillous epithelial cells are also present. The ultrastructural findings are consistent with the proliferation of neoplastic cells that have the capacity to produce all of the epithelial elements of the breast, i.e., ductal, acinar, and myoepithelial, and so recapitulate the complete ductal or acinar unit.