In 4 patients, an incidentaloma of the thyroid was found on 18-fluoro-deoxyglucose positron-emission tomography (FDG-PET). In the first patient, a 73-year-old man, a medullary thyroid carcinoma was discovered during the staging procedure ofa laryngeal carcinoma. In the second patient, an 81-year-old woman, a follicular thyroid carcinoma was found as a result of a FDG-PET evaluation of an adenocarcinoma of the lung. In the third patient, a 64-year-old woman, a papillary thyroid carcinoma was found during dissemination investigation after curative removal of an adrenocortical carcinoma. The last patient, a 78-year-old man, was found to have a thyroid incidentaloma on FDG-PET scan during staging ofa recurrence of a gastrointestinal stromal tumour. Thyroid incidentalomas are present on 1.2-2.3% of FDG-PET scans. Further diagnostic work-up of these lesions by fine needle aspiration is warranted since up to 50% are malignant. However, whether these malignant thyroid lesions are relevant is not always clear. Treatment depends on the primary disease for which the FDG-PET scan was initially made. This requires good evaluation and discussion with the patient.