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Analysis of differentiation antigens on leukaemic blasts is routinely done for diagnostic purposes, i.e. determination of stage of differentiation and lineage assignment. Acute lymphoblastic leukaemias are also frequently characterized by a leukaemia-associated immunophenotype (LAIP), either the coexpression of differentiation antigens physiologically restricted to other stages of differentiation (asynchronous LAIP) or cell lineages (aberrant LAIP). We defined LAIP in 241 consecutive unselected B-lineage (n = 193) and T-lineage (n = 48) ALL by three-colour flow cytometry using directly conjugated monoclonal antibodies. The incidence of LAIP was found to be 91.7%. In 63% of patients two to six leukaemia-associated expression patterns were detected. In order to study the specificity of LAIP in a therapy-relevant setting, remission bone marrow samples from patients with B-lineage ALL were analysed for the expression of T-lineage-associated phenotypes on the normal bone marrow cells and vice versa. The frequency of all T-lineage LAIP+ cells and all aberrant B-lineage LAIP+ cells was <1% in regenerating bone marrow samples at different timepoints. The incidence and clinical significance of LAIP+ cells was studied in 196 remission marrows of 70 ALL patients (55 remaining in CCR, 14 with bone marrow relapse, one with isolated CNS relapse). The presence of >1% LAIP+ at two consecutive timepoints predicted 5/8 bone marrow relapses in B-lineage ALL. The occurrence of LAIP+ cells >1% in T-lineage ALL after induction therapy predicted relapse in 7/7 cases. In conclusion, flow cytometric detection of LAIP+ cells appears to be a powerful tool for the prediction of outcome in ALL. [ABSTRACT FROM AUTHOR]