Invasive thymoma with CD4+CD8+ double-positive T cell lymphocytosis.

Lymphocytosis in the peripheral blood is a very rare manifestation of thymoma. A 45-year-old man presented with a giant mediastinal mass involving pleural dissemination and peripheral T cell lymphocytosis. Biopsy of the mediastinal mass revealed invasive thymoma, and two-color analysis of peripheral lymphocytes showed a marked increase in CD4+CD8+ double-positive cells. Both thymoma and lymphocytosis were improved by a combination of chemotherapy and mediastinal irradiation.