A 77-year-old female with a history of hypertension was admitted to our hospital for cough lasting for 1 year. Computed tomography showed a 5x4x3 cm nodular anterior mediastinal mass. A complete surgical resection of the tumor was performed by median sternotomy. Macroscopically the mass 5x4x3 cm was surrounded by a thin capsule. Microscopically a tumoral tissue characterized by a proliferation of multiple epithelial nodules seperated by an abundt lymphoid tissue was seen. The surrounding lymphoid tissue contained a dense proliferation of small lymphocytes forming lymphoid follicles with prominent germinal centers. Immunohistochemically, the epithelial cells were positive for pancytokeratin and CK5/6, lymphoid follicles were positive for CD20 and few scattered lymphoid cells in and around the nodules were positive for CD 3 and CD5. Histopathologic diagnosis was micronodular thymoma.

It is important to differentiate these tumors from other types of thymomas because of their association with low grade lymphomas. A rare case of micronodular thymoma was presented and discussed with the literature.