Ferrokinetic measurement were performed in a total of 48 patients with myelodysplastic syndrome (MDS), 13 with hypoplastic MDS and 25 with aplastic anemia (AA). Forty seven % of patients with hypoplastic MDS progressed to acute leukemia, however none of the patients with AA progressed to acute leukemia. Sixty-nine% of the patients with MDS showed increased erythron transferrin uptake (ETU) caused by ineffective erythropoiesis. On the other hand, 85% of the patients with hypoplastic MDS and all patients with AA showed decreased ETU caused by reduced erythropoiesis. Positive correlation was observed between ETU and the erythroid precursor cells in the bone marrow. Bone marrow scintigraphy utilizing 99mTc-sulphur colloid showed peripheral expansion of active marrow in MDS patients and islands-like distribution in hypoplastic MDS and AA patients. Ferrokinetics and bone marrow scintigraphy demonstrated the difference between typical MDS and hypoplastic MDS. Hypoplastic MDS appears to be a distinct clinicopathologic entity.