The following diagnostic criteria are proposed to classify four clinical subtypes of HTLV-1 associated adult T-cell leukaemia-lymphoma (ATL): (1) Smouldering type, 5% or more abnormal lymphocytes of T-cell nature in PB, normal lymphocyte level (less than 4 x 10(9)/l), no hypercalcaemia (corrected calcium level less than 2.74 mmol/l), lactate dehydrogenase (LDH) value of up to 1.5 x the normal upper limit, no lymphadenopathy, no involvement of liver, spleen, central nervous system (CNS), bone and gastrointestinal tract, and neither ascites nor pleural effusion. Skin and pulmonary lesion(s) may be present. In case of less than 5% abnormal T-lymphocytes in PB, at least one of histologically-proven skin and pulmonary lesions should be present. (2) Chronic type, absolute lymphocytosis (4 x 10(9)/l or more) with T-lymphocytosis more than 3.5 x 10(9)/l, LDH value up to twice the normal upper limit, no hypercalcaemia, no involvement of CNS, bone and gastrointestinal tract, and neither ascites nor pleural effusion. Lymphadenopathy and involvement of liver, spleen, skin, and lung may be present, and 5% or more abnormal T-lymphocytes are seen in PB in most cases . (3) Lymphoma type, no lymphocytosis, 1% or less abnormal T-lymphocytes, and histologically-proven lymphadenopathy with or without extranodal lesions. (4) Acute type, remaining ATL patients who have usually leukaemic manifestation and tumour lesions, but are not classified as any of the three other types. A total of 818 ATL patients with a mean age of 57 years, newly diagnosed from 1983 to 1987, were analysed by this criteria. There were 448 males and 370 females, and 253 were still alive with a median follow-up time of 13.3 months from diagnosis, while 565 were dead with a median survival time (MST) of 5.4 months. MST was 6.2 months for acute type, 10.2 months for lymphoma type, 24.3 months for chronic type, and not yet reached for smouldering type. Projected 2- and 4-year survival rates were 16.7% and 5.0% for acute type, 21.3% and 5.7% for lymphoma type, 52.4% and 26.9% for chronic type, 77.7% and 62.8% for smouldering type, respectively. Distinct clinical features and laboratory findings of each clinical subtype are described.