Lymphocytosis in the peripheral blood is a very rare manifestation of
thymoma. A 45-year-old man presented with a giant mediastinal mass inv
olving pleural dissemination and peripheral T cell lymphocytosis, Biop
sy of the mediastinal mass revealed invasive thymoma, and two-color an
alysis of peripheral lymphocytes showed a marked increase in CD4+CC8double-positive cells. Both thymoma and lymphocytosis were improved by
a combination of chemotherapy and mediastinal irradiation.