The existence of leukemic-associated phenotypes has been suggested to be a valuable tool for the detection of minimal residual disease (MRD) in AML patients, as they would allow to distinguish leukemic blast cells from normal hematopoietic progenitors. The present study was designed to analyze in which proportion of AML patients the immunological detection of MRD is feasible, based on the presence of aberrant phenotypes that allow the distinction of leukemic from normal cells. For this purpose we have prospectively investigated the blast cells from 40 AML patients at diagnosis with a large panel of MoAb in double and triple staining combinations analyzed at flow cytometry, in order to detect aberrant phenotypes on blast cells (lineage infidelity, antigenic overexpression, and asynchronous antigenic expression, as well as aberrant light-scatter pattern). In the analysis of the 40 AML cases more than one blast cell subset, distinguished by its different antigenic expression, was detected in 85% of the patients: five different phenotypic blast cell subsets were observed in six cases, four in 13 patients, three subsets in three cases, and two in 12 patients; only six cases showed a homogeneous phenotypical blast cell population. Twenty-nine of the 40 AML cases analyzed (73%) showed the existence of at least one aberrant phenotype: in 15 cases the myeloid blast cells co-expressed lymphoid-associated antigens (CD2, CD5, CD7, and/or CD19)--lineage infidelity--; asynchronous antigen expression was detected in 25 patients (CD34+CD56+, CD34+CD11b+, CD34+CD14+, CD117+CD15+, CD33-CD13+, CD13-CD15+, HLADR + CD15 , HLADR-CD14+CD11b+ CD4+); seven cases displayed antigen overexpression (CD13, CD33, CD15, or CD14); and in 13 patients leukemic cells had an abnormal FSC/SSC distribution according to their phenotype. These results suggest that immunological methods for the detection of MRD based on the existence of aberrant phenotypes could be used in the majority of AML patients.