Eleven patients with acute myeloid leukemia (AML) were studied with a technique that simultaneously identifies cytogenetic abnormality and immunophenotype of the same mitotic cell. To determine the cell lineages with abnormal karyotypes, monoclonal antibodies in the alkaline phosphatase-antialkaline phosphatase (APAAP) detection procedure were used. The granulocytic/monocytic lineage was involved in the leukemic process in all 11 patients. In nine patients, we also detected abnormal karyotypes in the erythrocytic and/or megakaryocytic lineages. All four patients with secondary AML showed involvement of the granulocytic/monocytic, erythrocytic, and megakaryocytic lineages into the leukemic process, as compared with five of seven patients with de novo AML. One patient with trisomy 8 showed erythrocytic participation in the leukemic process, but in another the erythrocytic lineage had only normal karyotypes. Thus, in AML, the chromosome abnormalities apparently usually originate at the multipotent progenitor cell stage, since in addition to granulocytic/monocytic lineages, erythrocytic and/or megakaryocytic lineages were also involved. Some patients show involvement of granulocytic/monocytic lineages only, however, suggesting that the target cell belongs to a more mature committed progenitor cell stage.