Thirty-three patients with small cell carcinoma of the lung were treated surgically, and immunohistochemistry of the cell differentiations was examined in detail. The overall 5-year survival rate was 38% and the rates in patients with stage I or stage III were 57% and 11%, respectively (P less than 0.05). Survival rates in patients with the oat cell type and intermediate type were 24% and 44%, respectively, but with no significant difference. This carcinoma seemed to originate from primitive multipotential stem cells, i.e., those of a neuroendocrine or epithelial nature. Histochemically and immunohistochemically, argyrophilic granules and neuron-specific enolase, neuroendocrine markers, were detected more frequently in the oat cell type rather than in the intermediate type. In contrast, keratin, epithelial membrane antigen, and carcinoembryonic antigen, epithelial origin markers, were present more frequently in the intermediate type than in oat cell type. However, the difference was significant only in case of detection of argyrophilic granules and the carcinoembryonic antigen (P less than 0.05). Our current recommendation is that surgical resection should be done in the earlier stage in both subtypes. A more favorable prognosis can be expected when adjuvant chemotherapy is prescribed.