• Twenty-five Hürthle cell tumors of the thyroid gland, histopathologically classified into three groups—adenomas, carcinomas, and indeterminant—have been studied by DNA flow cytometry using archived, paraffin-embedded tissues. Tumor ploidy characteristics were correlated with patient follow-up and survival with the conclusions that (1) nuclear DNA ploidy alone does not distinguish benign from malignant Hürthle cell tumors; (2) diploid DNA Hürthle cell carcinomas behave far less aggressively than aneuploid Hürthle cell carcinomas; and (3) all patients with aneuploid carcinomas died of their disease or are alive with persistent carcinoma. (Arch Otolaryngol Head Neck Surg1988;114:520-521)