Seven cases of mucus-producing bronchioloalveolar carcinoma, which showed organoid differentiation simulating the gastric pyloric mucosa, were found among 176 cases of lung cancer. This type of adenocarcinoma, which corresponds to bronchioloalveolar carcinoma with mucus-secreting cells in the World Health Organization classification, characteristically formed papillary structures composed of two types of mucus cells: tall columnar cells in the upper portion of the papillary structure and more cuboidal cells in the lower portion. The former contained gastric surface mucous cell-type mucins that stained with galactose oxidase-cold thionine Schiff, whereas the latter possessed gastric gland mucous cell-type mucins specifically stained by paradoxical concanavalin A and were also positive for lysozyme and pepsinogen II by immunostaining. Chromogranin A-reactive tumor cells were also scattered among these tumor cells. This pattern of mucus-secreting cells, therefore, simulated the normal pyloric mucosa of the stomach.