A rare lung cancer consisting in part of small cell carcinoma of intermediate cell type and in part of well-differentiated papillotubular adenocarcinoma is described. Alcian blue-PAS staining was observed in the cytoplasm of the small cell carcinoma cells; the Grimelius argyrophil reaction was also positive in the cytoplasm of these cells. Electron microscopy revealed neurosecretory granules in the cytoplasm. At autopsy, a small cell carcinoma of intermediate cell type was found with both squamous features and gland formation. The cellularity and histological pattern of this tumor suggested the existence of a transitional pattern between small cell carcinoma of intermediate cell type, squamous cell carcinoma and adenocarcinoma. From the above findings, we think that small cell carcinoma including the intermediate cell type is derived from respiratory epithelial cells of endodermal origin with dedifferentiation of those cancer cells into neurosecretory cells.