Six cases of well-differentiated adenocarcinoma resembling fetal lung were studied histologically, immunohistochemically, and ultrastructurally, and compared with three cases of pulmonary blastoma. The six cases had no sarcomatous features, unlike the pulmonary blastoma. Individual tumor cells characteristically possessed clear cytoplasm with plentiful glycogen and were similar to the epithelial cells of the branching tubules in the pseudoglandular stage of the fetal lung, as well as the epithelial component of pulmonary blastoma. They also showed little differentiation toward mucous cells and ciliated cells. Although cells containing endocrine-type granules were not found ultrastructurally, a few tumor cells possessed the characteristics of endocrine cells, i.e., cytoplasm was immunohistochemically reactive with anticalcitonin and antigastrin-releasing peptide. Therefore, this type of adenocarcinoma is considered to have a histogenesis similar to that of pulmonary blastoma and may be a tumor with one-sided development of pulmonary blastoma showing only an epithelial component.