Functional tumor imaging using Fluorodeoxyglucose positron emission tomography (FDG-PET) is a new method in clinical oncology. The 18 FDG, is a glucose analog that accumulates in cells in proportion to the rate of glucose metabolism, and increased carbohydrate metabolism has been recognized as a feature of malignant cells versus normal cells. In addition, it permits the detection of metastases or synchronous tumours not discovered by anatomic imaging. Although detection of the primary site of disease is usually accomplished well with conventional techniques, the performance of FDG-PET may be useful to determine tumours that are not clinically evident. The authors describe a case of early detection of synchronous thyroid carcinoma by FDG-PET in a young patient opereted on for a malignant melanoma on his arm.