The value of radioiodinated metaiodobenzylguanidine (MIBG) in imaging thyroid medullary carcinoma (MTC) was investigated (18 studies) in 12 patients with proven MTC. Calcitonin (CT), carcinoembryonic antigen (CEA) and tissue polypeptide antigen (TPA) were also determined. The patients were divided into two groups. Group I comprised 7 patients who had relapsed; two of them were also studied before total thyroidectomy. In the 2 cases studied before total thyroidectomy 123/131I-MIBG imaged the primary tumor (partially) and the residual tumor involved lobe. The residual/recurrent tumor present in 4 and some of the remote metastases in 3 out of 5 were detected. Group II includes 5 patients studied postoperatively with no evidence of disease. A residual tumor in one of the 2 patients without evidence of disease on the basis of conventional diagnostic modalities but with elevated tumor markers was visualized; the outcome was correctly negative in 3. One patient underwent treatment with 131I-MIBG. A total dose of 27.1 GBq (733 mCi) was given. Relief of pain and partial regression of the lesions was achieved.