99mTc-penta-dimercaptosuccinic acid (DMSA) is a recently developed radiopharmaceutical which has been proven to be useful in the diagnosis and follow-up of Medullary Carcinoma of Thyroid (MCT). In this study we evaluate this radiopharmaceutical in 10 patients (pts) with histopathologically proven MCD and in one patient with pheocnromocytoma who was suspected of MCT. Three pts with primary tumor were imaged before surgical resection and showed intense uptake in the neck region. Four pts who were imaged after surgical resection, revealed no abnormal uptake in the neck regions. In all pts simultaneous serum calcitonin levels were also measured. The metastatic foci in cervical, supraclavicular, parasternal lymph nodes, and in lung, spine and ribs showed increased activity in the scintigrams. These lesions also were confirmed with other modalities such as chest X-ray, computerized tomography, magnetic resonance imaging and fine needle aspiration biopsy (NAB). In three of the 4 pts who had liver metastases the 99mTc-penta-DMSA scintigraphy did not show the metastatic foci and were accepted as false negative results. The reason was probably due to the size of those lesions which were less than 1.5 cm measured by ultrasonography. In a patient who had pheochromocytoma the test was negative and NAB confirmed this result. Overall the test showed 84 % sensitivity and 89 % specificity. With these findings we conclude that 99mTc-penta-DMSA scintigraphy is a sensitive and specific test and significantly contributes to the diagnosis and follow-up of pts with MCT.