Studies were performed to obtain evidence of the differentiating and hematopoietic potential of marrow clone-original cells in patients with myelodysplastic syndromes (MDS). First, results of correlation analysis between bone marrow clonal cells and blast percentages for a total of 60 MDS showed that almost all cases had higher clonal (mean 50.1%) than blast proportion (mean 7.0%) (p < 0.001). In contrast, in 16 AML patients, mean clone/blasts disparity was nearly zero. Secondly, the amount of clone-original individual cells were defined in mature hematopoietic cells, mean 47.9% in segmented granulocytes, 47.1% in orthochromatic normoblasts and 37.8% in mature megakaryocytes. In addition, FISH examination showed approximately similar proportions of clonal cells in peripheral blood and marrow for all 22 tested cases (mean 39.1% in blood vs 49.8% in marrow). Moreover, the neutrophils in MDS peripheral blood oxidized dihydrorhodamine123 (DHR) nearly the same as neutrophils in a normal donor's circulation, while obviously poor function was observed in neutrophils from AML blood. Of note, research on clonality had an unexpected outcome as most of the typical morphological dysplastic cells possessed normal karyotypes. These findings lead to the proposal that the biological features of MDS clones are distinctive from those of AML clones.