In Down syndrome, acute megakaryoblastic leukemia (AMKL) occurs frequently during the first 4 years of life and is usually preceded by a period of myelodysplasia (MDS), often associated with chromosomal abnormalities. Archival peripheral blood and/or bone marrow films of six patients with Down syndrome and MDS whose leukemic cells contained monosomy 7 or trisomy 8 were studied to determine whether the abnormal precursors produce mature cells in vivo. Using fluorescence in situ hybridization (FISH) of interphase nuclei with chromosome-specific centromere probes for either chromosome 7 or 8, we were able to determine which cells had one, two, or three signals indicative of one, two, or three no. 7 or 8 chromosomes. In five patients with trisomy 8, 80% to 100% (94.5% +/- 6.2%) of the megakaryoblasts had three signals using a chromosome 8 probe; in one patient with monosomy 7, 96.5% of the megakaryoblasts had one signal using a chromosome 7 probe. In all six patients, the myeloid and lymphoid series did not have evidence of the chromosomal abnormality present in the blasts. In three of five patients with trisomy 8, three signals were observed in 27%, 33%, and 41% of normoblasts, respectively. These data are evidence that the abnormal cell in MDS is a progenitor cell with the potential of forming cells of megakaryocyte and erythroid lineages.