Signet ring cell carcinoma is a variant of adenocarcinoma and has been rarely reported in the lung as a primary site. Recently, we experienced two cases of primary signet ring cell carcinoma in the lung without any other extrapulmonary lesion. Sputum cytology was performed and the tumor cells which have eccentrically located nuclei and abundnat mucinous cytoplasm were dispersed in diffuse sheets. On resected specimen, the signet ring cells occupied about $50{sim}80%$ of all tumor cell nests. Histochemical staining revealed that the mucin produced by tumor cells was mostly carboxylated acid mucins. Ultrastructurally, the tumor cells contained variable sized membrane-bound mucin granules with weak central osmilophilic density and showed numerous surface microvilli, which represented that tumor cells arose from bronchial epithelial cells. In general, this tumor has diffusely infiltrative nature and the prognosis is fatal due to widespread metastasis before clinical discovery.