Introduction: Medullary thyroid cancer (MTC) is a neuroendocrine tumor which originates from the parafollicular cells. The prognosis of MTC is poor and thyroidectomy can be effective. Castleman's disease or angiofollicular lymph node hyperplasia is an uncommon benign lymphoproliferative disorder. The most common form is hyaline vascular type which is usually asymptomatic and surgical resection often cures it.

Case Report: The case was a 38-year-old male with a thyroid mass resected by surgery. The pathology of the mass was MTC. He returned with dyspnea and imaging revealed a mediastinal mass that was Castleman's disease as reported by pathology. MTC lesions can metastasize to regional lymph nodes but the pathology of the mediastinal mass was not metastatic.

Conclusion: According to the literature, medullary thyroid cancer and Castleman's disease are not predisposing factors for each other up to now. This is the first case with simultaneous occurrence of these two diseases that might help to find a new etiology for them.