The adult T-cell leukemia/lymphoma (ATLL) syndromes comprise neoplasms that arise in peripheral lymphoid tissues but a high frequency present with blood involvement mimicking T-cell leukemia. Clinically ATLL is sub-classified into four groups: acute, lymphomatous, chronic and smoldering. ATLL is etiologically linked to the human T-cell lymphotropic virus type I (HTLV-I). The diagnosis of ATLL is based upon a combination of characteristic clinical manifestations, morphological and immunophenotypic changes of the malignant cells, in addition to the confirmation of HTLV-I infection. ATLL is an aggressive malignancy with a median survival of less than 12 months and no successful treatment yet available. Patients are either refractory or only transiently respond to chemotherapy or purine analogues. Smoldering and chronic ATLL pursue an indolent course and survival for years until the disease progresses and becomes refractory to therapy. The major causes of death in ATLL are opportunistic pulmonary infections and progressive disease, often in association with hypercalcemia.