Signet ring cell carcinoma of lung is an unique variant of mucin producing adenocarcinoma which is characterized by abundant intracellular mucin accumulation. Only a few cases of primary signet ring cell carcinoma of lung have been reported in the world wide literature. And we have, recently experienced one case of primary signet ring cell carcinoma of lung. A 55 years old man was evaluated for paralysis of lower extremities and was found to have lung cancer in the left upper and lower lobe with pleural, multiple spinal, bone and liver metastases. Signet ring tumor cells were revealed by cytologic examination of pleural fluids. And there were no evidence of signet ring cell carcinoma of other organs. Primary signet ring cell carcinoma of lung seems to have an aggressive behavior and therapeutic modalities could be different from those for signet ring cell carcinomas from other organs. Therefore it is important to separate primary signet ring cell adenocarcinoma of lung from metastatic tumors.