21-year-old man found to have an incidentally discovered anterior mediastinal mass during workup for syncope. It was over 5 cm. MRI was performed, which showed relatively benign-appearing features consistent with a likely thymoma. This was subsequently resected. Histomorphology showed lymphoid follicular hyperplasia, regressed germinal centers, and dendritic cell meshworks with onion skinning and twinning, consistent with hyaline vascular variety of Castleman's disease. It is unicentric.