Peripheral T-cell lymphomas and leukemias have distinct histological and cytological characteristics although there are a variety of histological patterns even in a single lymph node. Histologic features are altered by the progression of the disease. There is no significant association between histology and prognosis. The variations in histologic features and clinical manifestations of peripheral T-cell lymphomas/leukemias can now be related to the presence of the retrovirus ATLV, and anti-ATLV antibodies as well as to the immunophenotypes of the lymphoma cells utilizing monoclonal antibodies. Changing concepts of peripheral T-cell lymphomas/leukemias are discussed.