Excision of the anterior mediastinal mass was done. Histopathologic examination revealed nodal tissue with proliferation of follicles characterized by regressively transformed germinal centers. There is tight concentric layering of lymphocytes at the periphery of the follicles resulting in an onionskin-like appearance. The follicles showed marked vascular proliferation and hyalinization of the germinal centers. The intrafollicular stroma is otherwise prominent, with numerous hyperplastic vessels and admixture of plasma cells, eosinophils, and immunoblasts. Associated vascular proliferation is also present in the surrounding soft tissue. The histologic features were compatible with Castleman disease, hyaline vascular type (angiofollicular lymph node hyperplasia).