Adult T cell leukaemia-lymphoma (ATL) was first discovered and reported in Japan, where it has a high incidence in the south-west region. The first human retrovirus HTLV-I (human T-cell lymphotropic virus type I) is considered to be related to its aetiology. In ATL endemic areas, HTLV-I carriers form a fairly high percentage of the population, even among healthy individuals. ATL shows diverse clinical features. It can be divided into four subtypes: acute, chronic, smouldering and lymphoma type. ATL cells originate from the CD4-positive subset of peripheral T cells; they show a characteristic notch in the nucleus and a tendency to lobulation. ATL resists chemotherapy, and patients with acute and lymphoma types have a fairly poor prognosis. A definite diagnosis of ATL is made by documenting the presence of HTLV-I proviral DNA in the DNA of tumour cells. HTLV-I infection is caused by transmission of live lymphocytes via three routes (from mother to child, from males to females, and by transfusion). Familial occurrence of ATL is frequently seen. HTLV-I infection is seen in other countries, but its incidence is highest in Japan. 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 disease, opportunistic lung infection, cancer of other organs, monoclonal gammopathy, chronic renal failure, strongyloidiasis, non-specific dermatomycosis, HTLV-I-associated lymphadenitis, HTLV-I uveitis and HTLV-I-associated myelopathy-tropical spastic paraparesis (HAM/TSP).