During the past two decades immunophenotyping has yielded significant new information regarding the biological heterogeneity of ALL and has provided a solid basis for a biologically oriented and reliable classification of this disease. At present, lineage commitment of acute leukaemias can be achieved in more than 98% of cases by applying a standardized panel of mAbs to pan-B-cell (CD19, cyCD22), pan-T-cell (cyCD3, CD7) and pan-myeloid antigens (CD13, CD33, MPO) that are expressed either on the surface or in the cytoplasm of the earliest progenitors of the respective cell lineage. Further subclassification of ALL based on the analysis of antigens more closely associated with different maturational stages of B- and T-cell lineage has proven useful for the identification of biologically and clinically distinct entities in both B-cell precursor and T-lineage ALL. Immunophenotyping in about 2800 patients recruited for the German multicentre trials has shown that children and adults differ markedly in frequency distribution of immunological subgroups, with a higher adult incidence of immature B-cell precursor (i.e., pre-pre-B ALL) and T-lineage ALL immunophenotypes (i.e., pre-T ALL). Detailed immunological analyses using a broad panel of mAbs have recently documented typical ALL cases inappropriately expressing myeloid antigens (My+ ALL) as well as morphologically/cytochemically defined acute myeloid leukaemia (AML) with lymphoid-associated markers (Ly+ AML). Based on our own results and a critical review of published data, leukaemic blasts in 5-20% of ALL patients disclose My+ ALL, whereas a coexpression, mostly of T-cell-associated antigens, can be identified in 10-25% of AML cases.(ABSTRACT TRUNCATED AT 250 WORDS)