Adult T-cell leukemia (ATL), a disease entity first described by Takatsuki et al., is endemic in southwestern Japan, the Caribbean Islands, and in some parts of Africa. ATL patients are classified into four subtypes according to the clinical picture: acute, chronic, smoldering, and lymphoma type. The diagnosis of ATL is made from the characteristic clinical findings, the detection of serum antibodies to HTLV-I, and when necessary, the confirmation of monoclonal integration of HTLV-I proviral DNA in cellular DNA of ATL cells. Recently, diagnostic criteria for clinical subtypes of ATL were proposed by the Lymphoma Study Group in Japan: 1) smoldering type, normal lymphocyte level, no hypercalcemia, lactate dehydrogenase (LDH) value 1.5 times the upper limit of normal or lower, no lymphadenopathy, no involvement of liver, spleen, central nervous system (CNS), bone or gastrointestinal tract, and no ascites or pleural effusion: 2) chronic type, absolute lymphocytosis with T-lymphocytosis of greater than 3 x 10(9)/1, LDH value twice the upper limit of normal or lower, no hypercalcemia, no involvement of CNS, bone, or gastrointestinal tract, and no ascites or pleural effusion: 3) lymphoma type, no lymphocytosis, 1% or less abnormal lymphocytes, and histologically-proven lymphadenopathy: 4) acute type, remaining ATL patients who are not classified as any of the above types. Infection with HTLV-I is a direct cause of ATL. Furthermore, infection with this virus can indirectly cause many other diseases via the induction of immunodeficiency, such as chronic lung diseases, opportunistic lung infections, cancer of other organs, monoclonal gammopathy, chronic renal failure, strongyloidiasis, non-specific dermatomycosis, non-specific lymph node swelling, HTLV-I associated myelopathy (HAM/TSP), and HTLV-I uveitis.(ABSTRACT TRUNCATED AT 250 WORDS)