In a retrospective review of all patients who visited our hospital between January 1997 and December 2001, we identified 22 with mediastinal lymphoma or mediastina granulocytic sarcoma. They represented 24.2% of the 91 patients with mediastinal tumors. Histology revealed 6 cases of diffuse large B-cell lymphoma, 6 of lymphoblastic lymphoma, 6 of Hodgkin's disease, 2 of granulocytic sarcoma, and 1 of lymphoplasmacytic lymphoma. More than 1/3 of the tumors had highly aggressive histological pictures. Immunocytochemical analysis of cell surface markers by flow cytometry was very useful for reaching a definitive diagnosis of these tumors. In two cases, definitive diagnosis could be obtained only by flow cytometric examination of pleural or pericardial effusion. Careful attention should be paid to the relatively high incidence of hematologic malignancies in mediastinal tumors. Early and accurate diagnosis of these tumors is essential because some of these patients require immediate treatment by hematology specialists.