We report a case of heterotopic breast epithelial inclusion of the heart incidentally found on a native heart in a 73-year-old man who received orthotopic heart transplantation for ischemic cardiomyopathy. The lesion could not be recognized on gross inspection. Histologic sections from the left anterior atrium to interatrial septum showed focally microcystic ductal/tubular structures lined by a biphasic pattern of cuboidal to columnar apical epithelial cells with an outer layer of flattened basal cells. These glandular structures were arranged in vaguely lobular and focally infiltrative patterns in the epicardium and interstitium. No architectural or cytologic atypia or mitotic or apoptotic figures were seen. The apical epithelial cells were immunoreactive for pankeratin, cytokeratin (CK) 7, estrogen receptor, progesterone receptor, gross cystic disease fluid protein-15, and negative for CK20, calretinin, Wilms' tumor suppressor gene (WT1), CD31, suggestive of mammary epithelial differentiation. The basal cells were immunoreactive for pankeratin, CK7, CK5/6, D2-40, smooth-muscle actin and focally S100, suggestive of myoepithelial differentiation. Although the heterotopic breast tissue on the skin along the milk line is well recognized, it has not been described to involve internal organs including the heart.