BACKGROUND: Microscopic examination of the bone marrow (BM) smear has been a major method for the diagnostic and post-therapeutic evaluation of hematologic disease but is laborious and imprecise due to small number of cells counted. Recently, automated reticulocyte counting is available by the automated hematology analyzer. We analyzed the bone marrow aspirates using Coulter GEN S (GEN S) automated hematology analyzer and compared the results with those by the microscopic examination. METHODS: Total nucleated cells (TNC), leukocyte subpopulations, red cell count, hemoglobin and reticulocyte indices of the peripheral blood (PB) and the BM aspirates, were measured by GEN S in 392 samples including 142 normal control samples. Differential counts by microscopic examination of Wright stained BM films were used as a reference. RESULTS: TNC of the BM obtained by automated hematology analyzer correlated with the BM cel-lularity estimated by microscopic examination (r=0.587, P=0.000). The differential counts of neutrophils and monocytes correlated between these two methods (r=0.582, P=0.000, r=0.309, P=0.000). In acute leukemia, TNC of the PB and the BM, and the BM lymphocyte fraction were increased and the BM neutrophil fraction was decreased. In chronic myelogenous leukemia, TNC of the PB and the BM were high but distribution of leukocyte subpopulations was normal. In normal control group, the number of erythroid precusors correlated with the percentages of reticulocyte in the PB (r=0.425, P=0.000), and in patients with increased erythropoiesis, it showed strong correlation with immature reticulocyte fraction (IRF) of the PB (r=0.708, P=0.033). In aplastic anemia, IRF of the PB was reversely correlated to hemoglobin level, but in myelodysplastic syndrome, reticulocyte indices of the PB and the BM had no correlation with hemoglobin level. In patients with increased erythropoiesis, the percentages of reticulocyte in the PB were increased and those of the BM were decreased in proportion to reduction of hemoglobin level in the PB. CONCLUSIONS: Analysis of the BM aspirates using automated hematology analyzer will be useful in screening of pathological hematologic diseases and in estimating the bone marrow cellularity objectively before those by the microscopic examination. In anemia, this study could provide an additional information to evaluate the ineffective hematopoiesis using reticulocyte indices of the PB and the BM.