 Adult T-cell leukemia ATL is and aggressive characterized by the presence of cluster of differentiation 4 CD4cluster of differentiation 25 CD25-expressing T cells interleukin-2 IL-2R expressing in the peripheral blood and in lymphoid and other tissues. Denileukin diftitox OntakRegistered Trademark is a genetically engineered fusion protein that targets IL-2-expressing malignancies. Denileukin diftitox interacts with the IL-2R on the cell surface, is internalized via endocytosis, and inhibits cellular protein synthesis, resulting in cell death within hours to days. The objectives of this study are to determine the clinical response to Denileukin diftitox of patients with adult T-cell leukemia ATL and the safety of Denileukin diftitox in those patients. 