Simultaneous involvement of the anterior mediastinum by thymoma and B-cell chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL), to our knowledge, has not been previously reported. We describe a composite tumor composed of thymoma and CLL/SLL incidentally discovered in a 62-year-old man who had no history of malignant diseases or immunologic disorders. The preoperative peripheral blood specimen showed a normal complete blood cell count and differential count. The diagnosis was established by histologic examination and immunophenotypic studies of the surgically excised anterior mediastinal mass. Postoperatively, bone marrow aspiration and biopsy specimens showed morphologic evidence of CLL/SLL, and the presence of neoplastic cells in peripheral blood and bone marrow was confirmed by flow cytometry immunophenotypic analysis.