Adult T-cell leukemia (ATL) was first reported in Japan, where it has a high incidence in the southwestern region. The retrovirus, human T-lymphotropic virus type I (HTLV-I), is the causative agent of ATL. In ATL-endemic areas, the rate of HTLV-I carriers is high. A definite diagnosis of ATL is based on the presence of HTLV-I proviral DNA in the tumor cell DNA. ATL cells originate from the CD4 subset of peripheral T cells. ATL shows diverse clinical features but can be divided into four subtypes:acute, chronic, smoldering, and lymphoma type. Chemotherapy is not effective; the acute and lymphoma types have a poor prognosis. Familial occurrence of ATL is common. HTLV-I infection is caused by transmission of live infected lymphocytes from mother to child, from man to woman, or by blood transfusion. Infection with HTLV-I can lead to other diseases, including HTLV-I-associated myelopathy/tropical spastic paraparesis and HTLV-I uveitis.