Expression of TfR on the cell surface is known to be regulated by the cellular iron content. We previously reported an inverse correlation between mean number of TfR expressed and stainable iron granules (SIGs) of erythroblasts (EBLs) in normal subjects. In MDS, EBLs contain an increased number of SIGs, though the mechanism of this phenomenon is not well understood. In this report, expression of surface transferrin receptor (TfR) on bone marrow EBLs was examined in comparison with the number of SIGs in 11 patients with myelodysplastic syndrome (MDS) and 11 patients with acute leukemia (AL). The mean numbers of TfR from MDS (5.56±1.35x10(5)) and AL (5.73±2.25x10(5)) were not different from those of normal subjects (5.00±1.80x10(5)). However, in MDS the mean number of TfR from 8 patients (72%) was similar to the normal level in spite of the increased mean number of SIG. Four of the 11 patients with AL showed a similar change such as MDS in TfR, but notably 2 of those 4 patients were AL transformed from MDS. There is a correlation (r＝0.61) between TfR and MCV in MDS different from that in healthy subject. These findings suggest that TfR expression on EBLs, from MDS may escape from the regulation by the cellular iron content as iron-TfR regulation disturbance.