Acute leukaemias have traditionally been classified according to the nature of the predominating cells as judged by cytomorphology and cytochemistry. A codification of this classification into L1-L3 subdivisions for lymphoblastic cases (ALL) and M1-M7 for myeloid cases (AML), proposed by the FAB group of haematologists, has been used extensively in the past decade. Some criticisms of this codification are presented and other approaches to classification are discussed. Among these are included the potential importance of multiple lineage expression, measurements of cell differentiation using cytochemical criteria, the relevance of isoenzyme biochemistry of leukaemic cell populations, their surface antigenic differences as demonstrated by the use of monoclonal antibodies and the growing contribution of cytogenetics. The development and complexity of combined classifications which attempt to incorporate most of these features is illustrated. A simplified classification into broad groups is proposed, ALL being divided by surface antigenic and cytogenetic criteria and AML by morphology and cytochemistry. These groups may be further elaborated as appropriate.