Three patients with T-cell lymphoblastic lymphoma and peripheral blood eosinophilia are reported. At the time of diagnosis, all patients had lymphadenopathy, and one had a mediastinal mass. Lymph node biopsies revealed lymphoblastic lymphoma admixed with a variable number of mature eosinophils. Immunophenotypic studies demonstrated that each lymphoma had an immature T-cell immunophenotype. Bone marrow biopsies were hypercellular with myeloid hyperplasia and eosinophilia but were negative for lymphoma. All patients received multiagent chemotherapy; one patient achieved a complete remission, and two patients had partial remissions. All patients subsequently developed a myeloid malignancy. Two died of acute myeloid leukemia within 18 months of the diagnosis of lymphoblastic lymphoma. The third patient relapsed with a lymphoma that had histologic and immunophenotypic features of both T-cell lymphoblastic lymphoma and granulocytic sarcoma and also developed a poorly defined myeloproliferative disorder. These findings suggest that T-cell lymphoblastic lymphoma associated with eosinophilia may represent a distinct clinico-pathologic entity with a high risk of subsequent myeloid neoplasia.