The purpose of our article is to describe the immunohistochemical findings of intrathyroidal epithelial thymoma/carcinoma showing thymus-like differentiation (ITET/CASTLE) of the thyroid in detail, to clarify the difference between ITET/CASTLE and thymic lymphoepithelioma-like carcinoma (LELC), and to discuss the pathogenesis of ITET/CASTLE. We immunohistochemically examined five ITET/CASTLE and eight LELC cases. All of ITET/CASTLE cases were strongly positive for CD5, P63, high-molecular-weight cytokeratin and B-cell CLL/lymphoma-2. Carcinoembryonic antigen-positive carcinoma cells were found in four ITET/CASTLE cases. Neuroendocrine marker-positive carcinoma cells were scattered in all cases. Immunohistochemical findings in thymic LELC were essentially similar to those in ITET/CASTLE, but the sensitivity was different. There is a possibility that ITET/CASTLE and thymic LELC are not the quite same disease entity. We think that ITET/CASTLE is derived from ectopic thymus, but not related to solid cell nests.