When oncologists diagnose patients suspected of lymphoid malignancy, it is important to consider the possibility of adult T-cell leukemia/lymphoma (ATL) with a routine check for serum human T-cell lymphotropic virus type 1 (HTLV-1) antibody. The following points are essential for the diagnosis of ATL: (1) cytologically or histologically proven peripheral T-cell malignancy, and (2) positivity for anti-HTLV-1 antibody. When a patient is diagnosed with ATL, it is important to make an accurate diagnosis of clinical subtype in order to make appropriate treatment decisions. For patients with smoldering or chronic type ATL, close observation with careful monitoring for opportunistic infections is recommended. For patients with the acute or lymphoma type requiring therapy, enrollment in a clinical trial is recommended. When there is no active trial or the patient is ineligible for a trial, we recommend intensive chemotherapy used for aggressive non-Hodgkin lymphoma such as the LSG15 regimen (VCAP-AMP-VECP) based on a recent phase III study. Because most patients with ATL are not curable with current chemotherapy regimens, it is reasonable to consider the applicability of allogeneic stem cell transplantation inpatients who show responses to chemotherapy. For relapsed or refractory patients, enrollment in a new-agent trial should be considered in addition to stem cell transplantation.