In order to clarify the significance of the nuclear DNA ploidy pattern in primary lung cancer, recent reports of flow cytometric analysis of nuclear DNA are discussed. It has been reported in many institutes, that nuclear DNA ploidy pattern was a significant and an independent postsurgical prognostic factor in lung cancer, statistically. Diploidy tumors showed a better post-surgical outcome than aneuploid tumors. This fact is more evident in stage I tumor than in advanced cases and was more in squamous cell carcinomas than in adenocarcinomas. Some papers reported that tumors with a high DNA index showed more invasive character than that with a low DNA index with flow cytometric and histopathological examination. As the percentage of aneuploid tumor was much higher in primary lung cancer than in other cancers, more detailed characterization is believed to be needed in aneuploid tumors by means of other oncological analysis, e.g. growth factors or expression of oncogene products.