Five cases of adenocarcinomas of the lung with predominantly endobronchial polypoid growth in large bronchi were reported. The histology of these cases was well-differentiated papillary adenocarcinoma, the tumor cells of which were columnar (4/5) and cuboidal (1/5). More than 70% of the volume of each tumor grew in the bronchial lumen with neighboring intraepithelial growth. Ultrastructurally the tumor cells were characterized by numerous mitochondria (5/5), some basal bodies (5/5), poorly developed cilia (1/5), and many dilated smooth endoplasmic reticular near the cell surface (2/5); these are features characteristic of ciliated bronchial epithelial cells. There were some mucus-producing cells (3/5), which might be differentiating toward goblet cells. Membrane-bound secretory granules with an electron-dense core or fingerprint appearance were also found (2/5); these resembled Clara cell granules. The above findings suggested that polypoid adenocarcinoma in large bronchi had the ability to differentiate toward bronchial or bronchiolar surface epithelial cells with or without mucus production.