A uniform system of classification and nomenclature of the acute leukaemias, at present lacking, should permit more accurate recording of the distribution of cases entered into clinical trials, and could provide a reference standard when newly developed cell-surface markers believed to characterize specific cell types are applied to cases of acute leukaemia. Proposals based on conventional morphological and cytochemical methods are offered following the study of peripheral blood and bone-marrow films from some 200 cases of acute leukaemia by a group of seven French, American and British haematologists. The slides were examined first independently, and then by the group working together. Two groups of acute leukaemia, 'lymphoblastic' and myeloid are further subdivided into three and six groups. Dysmyelopoietic syndromes that may be confused with acute myeloid leukaemia are also considered. Photomicrographs of each of the named conditions are presented.