Adult T-cell leukemia-lymphoma (ATL) is a peripheral T-cell malignancy, closely associated with human T-cell lymphotropic virus type I infection. Clinically, ATL is classified into four subtypes: acute, lymphoma, chronic and smoldering type. Although the prognosis of chronic and smoldering-type ATL is relatively good, that of patients with acute- or lymphoma-type ATL still remains extremely poor. Zidovudine/IFN-α therapy seems to be promising, although its efficacy has not yet been confirmed in well-designed prospective studies. High-dose chemotherapy with the support of autologous transplantation does not improve outcome. Allogeneic stem cell transplantation is promising and approximately 40% of aggressive ATL patients are expected to survive long-term, although transplantation-related mortality is as high as 40-50%. Stem cell transplantation using reduced-intensity conditioning is also effective and safer, with graft-versus-ATL and graft-versus-human T-cell lymphotropic virus type I effects observed after transplantation. Novel approaches including new agents such as purine nucleoside phosphorylase inhibitors and histone deacetylase inhibitors, or targeted immunotherapy using antichemokine receptor-4 antibody or dendritic cell/peptide vaccine are also warranted.