Surgical specimens from fifteen patients with intraductal papilloma were reconstructed three-dimensionally from semi-serial sections to visualize the intraductal distribution of papillomas. Our results showed two basically different papillomas. In five patients, the papillomas were single and originated in the large ducts such as the segmental or subsegmental duct, but did not involve the terminal ductal-lobular units (TDLU); this type corresponded to the so-called solitary papilloma. In the remaining ten specimens, the papillomas were multifocal three-dimensionally; each had a root in the TDLU and spread into the large ducts, suggesting its purely peripheral origin. In view of this striking difference, and of possible canceration of ductal peripheries, a nomenclature of peripheral vs central papillomas is proposed instead of the conventional multiple vs solitary. Duct papillomatosis, invariably situated within the TDLU, was shown to be a continuation of peripheral papilloma and was regarded accordingly as a prepapillomatous condition.