Multiple endocrine neoplasia type 2B (MEN 2B) is an inherited cancerous syndrome characterized by medullary thyroid carcinoma (MTC), adrenal pheochromocytoma, marfanoid habitus, and enteric ganglioneuromatosis. In this syndrome, a high frequency of persistent elevation of the serum calcitonin level, a sensitive marker for MTC, after total thyroidectomy has been reported, and the prognosis of such patients depends upon complete resection of recurrent MTC by repeated surgery. The authors performed iodine 123 metaiodobenzylguanidine ((123)I-MIBG) radio-guided navigation surgery for recurrent MTC in a 14-year-old girl with MEN 2B. She had undergone 4 neck operations, including total thyroidectomy at the age of 7 years. An intravenous injection of 100 MBq (123)I-MIBG was followed by the fifth surgery. At surgery, the cervical and upper mediastinal areas were filled with adhesional scar tissue, in which a gamma-scintillation probe conducted hot spots of isotope uptake by cancerous cells. Histopathology of resected specimens showed scattered nests of MTC cells corresponding to gamma-scintillation counts. Intraoperative (123)I-MIBG scanning is of substantial benefit for children with MEN 2B undergoing surgery for recurrent MTC.