Medullary carcinoma of the thyroid (MCT) is malignancy derived from the parafollicular cells (or C-cells) of the thyroid. It is usually sporadic, although it is familial in some cases. Several scintigraphic procedures can provide information regarding the primary and metastatic foci of the tumour. We performed whole-body scanning to establish the pathology of MCT using Tl-201, Tc-99(m)(V)DMSA and Tc-99(m)-MIBI in 14 patients, and found average sensitivities of 73%, 82% and 81%, respectively. Moreover, we also scanned three patients with Tc-99(m)-tetrofosmin and identified two of four pathological foci as well as residual thyroid tissue. The sensitivities of Tl-201, Tc-99(m)(V)DMSA and Tc-99(m)-MIBI were 100%, 100% and 85% in identifying lymphadenopathies; 40%, 50% and 71% for soft tissue foci; 100%, 100% and 100% for foci in pulmonary parenchyma; and 100%, 66% and 100% for recurrences in thyroid gland. Although Tc-99(m)(V)DMSA identified all bony metastases in three patients (100%), Tc-99(m)-MIBI detected only two of three foci (66%) and Tl-201 none. Tl-201, Tc-99(m)-MIB and Tc-99(m)-tetrofosmin accumulated in residual thyroid tissue, but Tc-99(m)(V)DMSA did not, as expected. We conclude that these agents were complementary, since they had different sensitivities in different tissues. The tumour-seeking properties of tetrofosmin are to be evaluated in a larger series. ((C) 1999 Lippincott Williams & Wilkins).