The author reports herein two cases of ductal adenoma of the breast with an emphasis on immunohistochemistry. Both cases (patient 1, 58-year-old woman; patient 2, 78-year-old woman) were clinically suspected as carcinoma, and core biopsies were ‘indeterminate’ or ‘suspicious for malignancy’. Excisional biopsy and wide excision were performed. Histologically, both cases were ductal adenomas composed of ductal epithelial cells and myoepithelial cells. Patient 1 had extensive apocrine metaplasia. Immunohistochemically, myoepithelial cells were noted in both cases; cytokeratin (CK) 14 and p63 were the most reliable myoepithelial markers, followed by CD10, α-smooth muscle actin and S100 protein. CK profile was as follows: positive expression of CK5/6, CK18, CK19, and high-molecular-weight CK, and negative expression of CK20. This CK profile was the same as that of non-tumorous ducts, suggesting that the CK profile does not alter in tumorigenesis. The tumor cells expressed p53 protein (case 1, positive cell percentage 5%; case 2, 7%), c-erbB2 (HER2/neu, 76%, 64%), CEA (5%, 0%), estrogen receptor (33%, 84%), but were negative for progesterone receptor. Ki-67 labeling was 5% and 3%, respectively. MUC apomucin expression was as follows: MUC1, 92%, 100%; MUC2, 0%, 0%; MUC5AC, 0%, 0%; and MUC6, 5%, 0%. Non-tumorous ducts expressed MUC1, but were negative for MUC2, MUC5AC and MUC6.