The radio-responsiveness of 31 oral squamous cell carcinomas were studied by flow cytometric (FCM) DNA analysis, comparing DNA ploidy and histogram with histological evaluation according to Shimozato-Oboshi's classification.The clinical evaluation and WHO's histological grading did not agree with histological evaluation. The DNA analysis of the tumor was done before and after irradiation (12. 5 Gy-40Gy). The radio-responsiveness was histologically evaluated and the response rate was estimated after irradiation. The response rate was 67% in aneuploid tumors and 81%, in diploid tumors before irradiation. This suggests that ploidy pattern of non-irradiated tumor can not predict the radio-responsiveness. However, the response rate was 20% in aneuploid tumors and 93% in diploid tumors after irradiation. This suggests that ploidy pattern of irradiated tumors can predict radio-responsiveness. It was also found that the DNA histogram of irradiated radio-sensitive tumors resembles the histogram of normal tissues. These results concluded that the flow cytometric DNA analysis is useful in predicting the radio-responsiveness of tumors.