The frequency and degree of aneuploidy in 44 benign and I24 malignant thyroid neoplasms were analyzed by DNA flow cytometry. Single aneuploid cell populations were found in 72% of the undifferentiated carcinomas, 64% of the follicular carcinomas, 24% of the papillary carcinomas and in 24% of the follicular adenomas. Multiple aneuploid cell opulations were detected in 4% of the papillary and in 3 6 2 o f the follicular carcinomas but not in undifferentiated carcinomas. A low degree of aneuploidy was found in well differentiated papillary carcinomas (mean D N A index of aneuploid populations: DI = I . 17; SD f 0.09). Significantly higher values were found for aneuploid moderately differentiated papillary carcinomas (DI = I .46; SD k 0.29), well and moderately differentiated follicular carcinomas (DI = 1.61; SD f 0.33 and D I = 1.60; SD k 0.30, respectively) and undifferentiated carcinomas (DI = I .72; SD * 0.19). High DNA indices were also found in several follicular adenomas (DI = I .49; SD f 0.22). Comparison of the I 0-year survival rates of patients with moderately versus well differentiated papillary carcinoma (79 vs. 98 months, respectively) indicates that loss of differentiation and progression of aneuploidy in this tumour type is associated with more aggressive clinical behaviour. Similarly, the high frequency and degree of aneuploidy in undifferentiated carcinomas is in agreement with the very poor survival rate (0% at I0 years) in this group of patients. However, the occurrence of highly aneuploid adenomas and (nearkdiploid undifferentiated carcinomas does not point to a direct causal relationship between DNA-ploidy changes and clinical behaviour of these thyroid tumours.