Thymoma is associated with benign and neoplastic diseases. The authors report the concurrence of invasive thymoma and T-lymphoblastic leukemia/lymphoma in a 95-year-old man. The hematologic malignancy was suspected terminally, whereas the thymoma was discovered at necropsy. The lymphoblastic leukemia/lymphoma had a clonally rearranged T-cell receptor beta-chain gene and a mature thymocyte immunophenotype. No retroviral gene sequences (human immunodeficiency virus 1 and 2, and human T-cell leukemia virus 1 and 2) were identified by polymerase chain reaction and hybridization analysis. The association of thymoma with hematologic neoplasm is reviewed.