Elevated calcitonin (CT) levels after primary operation of the medullary thyroid carcinoma (MTC) are a reliable marker for persistence or recurrence of MTC, which first metastasizes in the neck or mediastinal region. The reliability of different localisation methods before reoperation in 28 patients with elevated CT levels was tested by comparing their diagnostic results with the actual finding at reoperation. The diagnostic procedures comprised ultrasonography of the neck, CAT-scan of the neck and mediastinum, selective venous catheterization with CT determinations, and fine needle biopsy. Due to the results of these tests 28 patients were reoperated 48 times. Histological evidence confirmed the presence of suspected tumor that had been diagnosed by: palpation 52%, ultrasonography 78%, CAT-scan 70%, selective venous catheterization 75%, fine needle biopsy 81%. Despite the fact that only 2 out of the 28 patients had normal CT levels postoperatively, the 5 year survival rate in reoperated patients (86%) improved compared to patients without reintervention (69%). For precise preoperative staging ultrasonography seems to be the most predictable and reliable method. The prognosis of MTC patients with elevated CT-levels in the follow-up period could be improved by frequent reoperations.