We investigated 5 MTC patients, 3 preoperatively for staging purpose, and 2 after surgery, during the follow-up, because of the persistence of elevated serum tumoral markers. FDG PET results were compared with conventional radiologic (US, CT scan, MRI) and scintigraphic non-invasive techniques (99mTc-MIBI and 99mTc-MDP scans). In all the 3 patients preoperatively studied, PET, as well as the other imaging modalities, detected the primitive tumor and the loco-regional lymphnode metastases. Furthermore, in one case, PET was the only technique that revealed an additional localization to the lungs. One false negative result was recorded with PET, as well as with the conventional imaging, in a MTC patient with a MEN II syndrome and with some liver micrometastases, 2 to 5 mm sized, showed only at laparotomy. PET was the only method capable of early visualizing a mediastinal relapse of the tumor in one of the 2 patients studied during the follow-up. This patient was re-operated and serum calcitonin levels became undetectable. On the basis of our preliminary results on MTC, PET with FDG seems to be an accurate, non-invasive technique, for staging purpose before surgery, and, during the follow-up for visualizing tumoral spread in patients with increased serum tumoral markers.