Thymic Hodgkin's disease (HD) shows some peculiar histological features different from nodal disease which are a result of the interaction with the specific thymic microenvironment. We describe the histological and immunohistochemical findings in three cases presenting as a primary thymic neoplasm both clinically and radiologically. Histological hallmarks were the prominent formation of epithelium-lined cysts, inflammatory changes, a marked proliferation of thymic epithelium in association with Hodgkin- and Reed-Sternberg (RS) cells and the occurrence of the nodular sclerosing subtype in all cases. The immunophenotype of the neoplastic cells was that of classical HD. They expressed CD30, CD15 and lacked CD45. In two cases CD20 expression was observed. All cases were negative for the latent membrane protein (LMP) of the Epstein-Barr virus (EBV). The accompanying inflammatory infiltrate was rich in mature T-cells, but also showed a significant number of B-cells with frequent formation of follicles and proliferation of follicular dendritic cells. Thymic HD develops in a microenvironment with features of thymic medulla as defined by the morphology and pattern of the proliferating epithelial cells and the mature immunophenotype of the admixed thymocytes. These findings, especially the CD20 positivity in Hodgkin and RS-cells, may point to the possible origin of thymic HD from medullary B-cells.