Myelodysplastic syndromes (MDS), especially refractory anemia (RA) are very heterogeneous diseases regarding their morphological, biological and clinical features. One important clinical problem is the difficulty of diagnosis. Soluble transferrin receptors (sTfRs) reflect the erythropoietic activity in the bone marrow (BM). To establish whether determination of serum sTfR could be useful for the differential diagnosis between RA and aplastic anemia (AA), we measured the serum sTfR concentrations, BM cellularity and BM erythroblast percentages in 14 untreated AA and 7 untreated RA patients. The serum sTfR levels of the RA patients (820.1 ± 402.8 ng/ml) were significantly higher than those of the AA patients (491.1 ± 195.2 ng/ml; p = 0.0207). However, the serum sTfR values of RA and AA patients also overlapped. A new index, the ‘sTfR-E index’ [the ratio of serum sTfR level (ng/ml) to BM cellularity (%) × BM erythroblasts (%)] is proposed, which is expected to reflect the number of transferrin receptors (TfR) on the cell membrane per BM erythroblast. The sTfR-E index values of the 7 RA patients (0.395 ± 0.234) were significantly lower than those of the 14 AA patients (2.669 ± 1.633; p = 0.0003). The sTfR-E index values of AA and RA patients overlapped only marginally. In conclusion, the sTfR-E index may be a useful new diagnostic tool to distinguish between AA and RA patients.