Adult T cell leukaemia/lymphoma was first identified by Japanese investigators in the mid 1970s. Distinctive characteristics include hypercalcaemia, metabolic bone disease, opportunistic infections and evidence of multiorgan involvement. The malignant cell has the surface phenotype of a T helper lymphocyte but functions as a T suppressor cell, and in leukaemic patients this cell usually has a unique multilobed appearance, which may aid in recognizing the disease. The overwhelming majority of patients with ATLL originate from the south-west Japanese archipelago, as well as the Caribbean basin and south-east USA. The geographic localization of this disease is the result of the endemic distribution of the human retrovirus (HTLV-I) which has been established as the cause of ATLL. Infection with this virus may result in no disease (asymptomatic carriers) or ATLL. While ATLL usually pursues an acute or subacute (prototypic) course, patients are also seen with 'chronic' or 'smouldering' disease. Over time, these more indolent variations may progress to the prototypic form. When aggressive, ATLL must be treated with intense combinations of cytotoxic drugs similar to those used to treat the more common B cell lymphoproliferative disorders. Even though half of the patients treated achieve a remission, the duration is usually brief and the overall actuarial median survival is only 11 months. In addition to recurrent disease, these patients frequently succumb to opportunistic infections.