Intaductal papillomas, papillary carcinomas, and normal mammary large ducts were studied by electron microscopy. The intraductal papillomas were divided into two subtypes: fibrous and glandular. Comparative ultrastructural analysis of these two subtypes revealed definitive differences in the nucleus, chromatin, and presence of myoepithelial cells. An irregularly shaped nucleus, aggregated heterochromatin, and prominent myoepithelial cells were the conspicuous features of the fibrous type which appeared to be almost absent in the glandular type. Moreover, the fibrous type possessed intact basal laminae, various differentiations of the cell surface, desmosomes, terminal bar, intrastromal projections of crab‐finger‐like appearance of the myoepithelial cells, and a two cell layer arrangement of epithelial cells. In these features, the ultrastructural appearance of the fibrous type closely resembled that of the cells of the normal mammary large duct. The glandular type, on the other hand, more or less lacked myoepithelial cells, well‐developed organelles and showed a euchromatin pattern in the nucleus. This type resembled the papillary carcinoma in its histological characters. An absence of basal laminae (infiltrating carcinoma only) and the presence of intracytoplasmic lumina were characteristic of papillary carcinomas, and these two features were not found in the intraductal papil