Human T-lymphotropic virus types I and II are retroviral infections that affect T-cells. HTLV-I was the first human retrovirus identified, isolated in 1978 from a patient with cutaneous T-cell lymphoma. A closely related virus, HTLV-II, was later isolated from a patient with hairy cell leukemia. Both viruses are highly cell associated, infect lymphocytes, and cause lifelong infections, although most of these infections remain asymptomatic. Approximately 2% to 5% of HTLV-I-infected individuals develop adult T-cell leukemia/lymphoma after a lag of 20 to 30 years. A smaller percentage develop a neurologic disease called HTLV-associated myelopathy or tropical spastic paraparesis. HTLV-II disease associations remain unclear. Both infections are thought to be spread through blood and sexual contact. FDA-licensed donor tests for HTLV infection are serologic screening assays for IgG antibody to HTLV-I and HTLV-II. FDA-licensed supplemental tests are also available.