Thyroid tumors with differentiation of both parafollicular and follicular epithelial cells are rare. The majority of those reported have been composed of a single cell population with combined features of the two cell types. We describe a 48-year-old man with a thyroid tumor composed of two discrete cell populations: thyroglobulin-positive papillary carcinoma intermixed with calcitonin-containing medullary carcinoma. The tumor metastasized to regional lymph nodes and maintained this composite differentiation. We propose that such tumors with two discrete intermingled cell populations be called composite thyroid carcinomas, as opposed to mixed thyroid tumors, which describes tumors with a single cell population that exhibits features of two cell types. Dual differentiation in thyroid neoplasms has been interpreted as indicative of a common stem cell origin. In this case, the presence of two mature, highly differentiated elements may suggest a proliferative response of two distinct cell populations to a common tumorigenic stimulus.