Three cases of oncocytic carcinoma of the breast observed in two women and one man are reported. One tumor was in situ and two were invasive. All three tumors were composed mostly of cells with "low-grade" nuclei and abundant granular eosinophilic cytoplasm. More than 70% of the neoplastic population in each case was immunoreactive with an antimitochondrion antibody. The presence of numerous mitochondria also was demonstrated at the ultrastructural level. Apocrine cells and oncocytes share similar morphologic features at the hematoxylin-eosin level; however, there are some differences that allow a confident distinction between these two cell types. Mitochondria in apocrine cells usually are in a perinuclear location and are not so numerous and diffusely dispersed as in oncocytes. In addition, apocrine cells display features of active secretory elements: prominent microvilli, well-developed Golgi complex, and electron dense secretory granules polarized toward the luminal pole; all these features were lacking in the three cases described. The cells constituting the present cases were not positive at the immunohistochemical and molecular levels for GCDFP-15/PIP mRNA, which are typical markers of apocrine differentiation. We suspect that mammary oncocytoma is a more common tumor than the meager number of reported cases suggests.