Leukemia-associated phenotypes have been suggested to be a valuable tool for the detection of minimal residual disease in acute leukemia patients, as they allow to distinguish leukemic blasts from normal hematopoietic progenitor cells. The aim of the present study was to analyze the proportion of acute leukemia patients (both with lymphoid and myeloid leukemias) in which the immunological detection of leukemia-associated phenotypes was convenient for the distinction of leukemic and normal cells. For this purpose we have studied the blast cells from 186 acute leukemia patients at diagnosis with a large panel of monoclonal antibodies by flow cytometry using double staining combinations. From aberrant phenotypes on blast cells we followed lineage infidelity (coexpression of myeloid markers in lymphoid leukemia cells and vice versa, as well as the simultaneous expression of both, T and B cell markers in one lymphoid blast cell) and asynchronous marker expression (simultaneous expression of early and late markers in one cell). One hundred and five of the 186 acute leukemia cases analyzed (56%) showed the presence of leukemia-associated phenotypes. In 41 of the 90 ALL cases followed (46%) and in 40 of the 96 AML cases studied (42%) lineage infidelity was observed. Asynchronous antigen expression was detected in 24 followed cases (13%). Evaluation of the cell marker density by means of calibration microbeads demonstrated abnormal mean channel immunofluorescence and molecules of equivalent soluble fluorescein for CD8 in two patients with T cell malignancies at diagnosis. Abnormal CD8 density might thus represent a characteristic feature of malignant CD8-positive T cell clone. Quantitative marker evaluation therefore seems to be another important mean for the detection of aberrant phenotypes on leukemia cells suitable for the detection of minimal residual disease.