Adult T-cell leukaemia or lymphoma is an aggressive malignant disease of mature activated T cells caused by human T-cell lymphotropic virus type I. Patients with this disease have a very poor outlook because of intrinsic chemoresistance and severe immunosuppression. In acute adult T-cell leukaemia, clinical trials in Japan show that although non-targeted combinations of chemotherapy improve response, they do not have a significant effect on complete remission and survival. Antiretroviral therapy with combination zidovudine and interferon alfa, which induces a high rate of complete remission and lengthens survival, should be the first treatment option in acute adult T-cell leukaemia. Patients with adult T-cell lymphoma might benefit from initial aggressive chemotherapy followed by antiretroviral therapy. To prevent relapse in all patients allogeneic bone-marrow transplantation when feasible, or additional targeted therapy, should be mandatory. Based on current pathophysiology, we discuss promising new drugs such as arsenic trioxide, proteasome inhibitors, retinoids, and angiogenesis inhibitors, as well as cellular immunotherapy.