Based on the results of histological and immunohistochemical observations of a large number of peripheral T cell lymphomas from China, England, Germany and Japan, histological and cytological morphology were correlated with immunophenotype, aetiological association with HTLV-1, and clinical behaviour to produce a working classification of the T cell lymphomas. This classification, based mainly on cytological criteria, divides the peripheral T cell lymphomas into tumours of low grade and high grade malignancy. Adult T cell lymphoma/leukaemia (ATLL) is caused by HTLV-1 and belongs chiefly to the high grade category. Some tumours are characterised by an admixture of other cells (epithelioid cells, follicular dendritic cells, etc) and structures (high endothelial venules, follicles), which may indicate the secretion of lymphokines by the tumour cells. Clear cells seem to be specific for T cell lymphomas and may occur in various types of peripheral T cell lymphoma.