Neuroendocrine tumors of the thymus bear many similarities to "carcinoids" and "oat-cell carcinomas" in other organs, and are clinicopathologically distinct from thymomas, thymic seminomas, and other primary tumors of this gland. They are associated with Cushing's syndrome or multiple endocrine neoplasia in 35% of cases, and are often locally aggressive. Approximately 30-40% of patients have distant metastases of their tumors that ultimately result in fatality, since the response of thymic neuroendocrine neoplasms to irradiation and chemotherapy is poor. Ultrastructural studies and immunohistochemical stains for chromogranin, protein gene product 9.5, and synaptophysin are effective tools for the diagnosis of thymic carcinoid and oat-cell carcinoma.