Fourteen human thyroid tumors were studied with 18F-fluorodeoxyglucose (FDG) imaging. The proliferative activity and DNA ploidy of the tumors was assessed by DNA flow cytometry. FDG accumulated in a Hürthle cell carcinoma, an anaplastic carcinoma and a thyroid lymphoma, but only slightly or not at all in the three papillary carcinomas studied. Three of the eight benign tumors also accumulated FDG, but two of these were selected for imaging because of a large number of proliferating cells in DNA flow cytometry. Two carcinomas with increased nuclear DNA content retained FDG, but a histologically benign follicular adenoma with DNA aneuploidy did not. We conclude that in addition to malignant thyroid tumors, histologically benign tumors may also accumulate FDG, and therefore the value of FDG scanning in the preoperative diagnosis of thyroid malignancy is limited.