Flow cytometric cellular DNA-RNA content analysis by acridine orange staining was conducted on surgical fresh specimens of primary lung carcinomas from 66 patients (31 squamous cell carcinomas, 34 adenocarcinomas and 1 large cell carcinoma). The frequency of aneuploid tumors was 84.6% among the tumors. RNA content (RNA Index) in the DNA aneuploid tumor much more significantly (p less than 0.05) increased than the DNA diploid tumor. Tumor doubling time in the DNA aneuploid tumor was significantly (p less than 0.05) shorter than in the DNA diploid tumor. In the patients with lung cancers that recurred within 1 years, recurrence of the DNA aneuploid tumor was higher than the DNA diploid tumor. It is evident from the above results that proliferative activity in the DNA aneuploid tumor increases much more than in the DNA diploid tumor. This in turn may induce early recurrence in patients with lung cancer.