We report a case of a medullary carcinoma of the left lobe of the thyroid gland that occurred in a 57-year-old woman. The patient had undergone surgery for treatment of a bilateral-functioning pheochromocytoma when she was 39 years old. A medullary carcinoma of the thyroid gland and/or a pheochromocytoma had also been diagnosed in other family members. The tumor was composed of cells arranged in nests and large sheets separated by fibrous stroma that contained amyloid deposits. Elongated cells with thin, branched cytoplasmic projections that were strongly reminiscent of sustentacular cells usually found in paragangliomas were seen among the neoplastic cells. Immunohistochemical study showed a diffuse positive reaction for calcitonin and low-weight keratins (CAM 5.2) in neoplastic cells, whereas the sustentacular cell-like cells were positive for S100 protein. The reaction for thyroglobulin was negative. Electron microscopy disclosed large numbers of typical neurosecretory granules in the cytoplasm of tumor cells. The sustentacular cell-like cells showed elongated cytoplasmic processes and lacked neurosecretory granules. We concluded that the finding of sustentacular cell-like cells in a medullary carcinoma of the thyroid gland made its differential diagnosis from paraganglioma more problematic.