To evaluate the biological malingnacy of differentiated thyroid cancer, we determined the nuclear DNA content in thyroid tumors by flow cytometry using paraffin-enbedded materials. The subjects were 80 patients with thyroid tumors. The thyroid tumors were follicular adenoma in 11 cases and thyroid cancer in 69 cases. Of the 69 cases of thyroid cancer, 42 were histologically classfied as papillary carcinoma, 18 as follicular carcinoma, 3 as medullary carcinoma and 6 as anaplastic carcinoma. DNA ploidy pattern and the percentage of proliferating phase cells were analyzed in relation to the prognosis and the following clinicopathological findings : age, gender, histological type, tumor size (t), extrathyroidal invasion (Ex), lymph node metastases (n) and distant metastases (M). DNA ploidy pattern correlated with histological type (p<0.005), but did not correlate with other clinicopathological findings. The percentage of prolipherating phase cells correlated with age (p<0.01) and the histlogical type (p<0.05), but did not correlate with other clinicopathological findings. The percentage of proliferating phase cells correlated with age (p<0.01) and the histological type (p<0.05), but did not correlate with other clinicopathological findings. The comulative survival rate (Kaplan- Meier) of differentiated carcinomas was worse in the aneuploid group than in the diploid group (p<0.0001). the percentage of proliferaging phase cells increased as the prognosis deteriorated. The results suggest that flow cytometric DNA analysis may be useful to evaluate biological malignancy of thyroid tumors.