AbstractMedullary thyroid carcinoma (MTC) is thought to develop from the parafollicular or C cells within the thyroid gland and, therefore, should not take up radioactive iodine. Recently, however,131I uptake has been reported in metastases of “medullary carcinoma” and, consequently, some authors have suggested that these tumors can be treated with radioactive iodine. On the basis of observations in 5 patients out of 11 with MTC, we would like to comment regarding this situation. In our 5 patients who were treated by total thyroidectomy, the diagnosis of medullary carcinoma was made by experienced pathologists using immunohistochemical staining methods (calcitonin and/or thyroglobulin), peroxidase‐antiperoxidase techniques, and electron microscopy. Evaluation of serum concentrations of calcitonin and serum thyroglobulin by radioimmunoassay was also performed. Data from our 5 patients provide evidence that patients with histopathological and immunohistochemical criteria for medullary thyroid carcinoma, and with high serum calcitonin levels, can take up131I. It is possible that these tumors represent poorly differentiated thyroid carcinomas with C‐cell metaplasia or mixed tumors originating from both follicular and parafollicular cells. Other neoplasms classified as “medullary cancer” seem to be unspecified thyroid cancers with C‐cell hyperplasia in the periphery of the tumor.