Eosinophilia is associated with a number of disorders including malignancies. A patient is described who had eosinophilia associated with adult T-cell leukemia/lymphoma (ATL) induced by human T-lymphotropic virus type I (HTLV-I). Both tissue and peripheral blood eosinophilia and high titers of HTLV-I antibody were present. The eosinophilia was most likely caused by the malignant cells producing one or more lymphokines. The patient has achieved a durable complete remission from combination chemotherapy. Because durable remissions in ATL are rare with any known therapy and eosinophilia has not previously been associated with ATL, it is possible that the tumor in this patient was derived from a T-cell subset not usually transformed by HTLV-I. ATL is another malignancy now known to cause eosinophilia.