Three patients with multiple metastases of thyroid carcinoma were studied with 18F-2-fluoro-2-deoxy-D-glucose (FDG) and iodine-131 (131I) imaging. Metastases that accumulated only FDG, only 131I, or both FDG and 131I could be demonstrated. Metabolic heterogeneity was seen between different metastases in all patients. The accumulation of FDG may also differ between different metastases of the same patient. The uptake of FDG in metastases was shown to increase parallel with their progression. Metastases of thyroid carcinoma that have ceased to accumulate 131I after treatment with radioiodine may still be demonstrated with FDG. Metastases that accumulate FDG, but not 131I, may behave more aggressively than metastases that accumulate 131I, but not FDG.