Objective: To show the value of positron emission tomography (PET) with 18-F-fluorodeoxyglucose (18-FDG) for the detection of metastases of differentiated thyroid carcinoma in selected patients. Patient histories: There were four patients, who had undergone total thyroidectomy for papillary (two) or follicular thyroid carcinoma (two). All patients had subsequent treatment with 131iodine. Three patients had an increasing serum concentration of thyroglobulin, one patient had antibodies against thyroglobulin. A diagnostic 131iodine scintigraphy was negative in two patients, and uncertain in two patients. Positron emission tomography was performed about 45 min after administration of 10 mCi 18-F- fluorodeoxyglucose. In three patients PET showed uptake in the cervical region, caused by lymph node metastases in two (confirmed by neck dissection) and recurrent tumor on the trachea in one patient (confirmed by surgery). In the fourth patient uptake of 18-FDG was seen in the neck and in both lungs. This led to discontinuation of treatment with 131iodine because the lung metastases did not accumulate 131iodine. Discussion: In selected patients with differentiated thyroid carcinoma with an increasing serum concentration of thyroglobulin, PET is an important diagnostic option when scintigraphy with 131iodine is negative or uncertain. In the four presented case histories, the results of PET led to a therapeutic decision: surgery in three patients and discontinuation of 131iodine in one patient. The development of guidelines for the use of PET in the diagnosis of recurrent thyroid cancer is discussed. (C) 2000 Elsevier Science B.V.