Adult T-cell leukemia: Abbreviated ATL. A malignancy of mature T lymphocytes (T cells) with its onset in adulthood caused by infection with the human T-lymphotropic virus (HTLV) and characterized by circulating malignant T-lymphocytes, skin lesions, lymphadenopathy (enlarged lymph nodes), hepatosplenomegaly (enlarged liver and spleen), hypercalcemia (high blood calcium), lytic ("punched out") bone lesions, and a tendency to infection. There are four categories of ATL, based on the aggressiveness of the disease -- smoldering, chronic, lymphoma, and acute.

The disease is also called adult T-cell leukemia/lymphoma and adult T-cell lymphoma. See also: Human T-lymphotropic virus.