Erythropoietic activity in patients with myelodysplastic syndrome (MDS) was evaluated by ferrokinetic measurements. Since the conventional plasma iron turnover of MDS patients increased with plasma iron levels after multiple blood transfusions, erythron transferrin uptake was chosen as a parameter of erythroid marrow activity. Although a correlation was shown between plasma iron level and plasma iron turnover (r = 0.50, 0.01 < p < 0.02), no correlation existed between the plasma iron level and erythron transferrin uptake (r = 0.25, p > 0.1). Erythron transferrin uptake, independent of plasma iron, was significantly higher in MDS patients than in normal subjects (110.6 ± 67.6 and 67.6 ± 18.8 μmol/l/dl, respectively; 0.01 < p < 0.02). An increased erythropoiesis occurring concomitantly with morphologically normal or increased erythroid cellularity was demonstrated in patients with MDS. The measurement of erythron transferrin uptake might be valuable as an accurate expression of erythroid activity in the hyperferremic state.