New automated blood cell analyzers, Coulter Counter Model S-Plus® series provide an index of red cell volume distribution width (RDW) or heterogeneity and a histogram display of red cell volume distribution. We evaluated the clinical significance of RDW by determining the normal range of RDW in healthy adults, children, and newborns and the change of RDW in pediatric patients with various hematologic diseases. The normal ranges of RDW in adults, children and newborns were 12.3±0.8, 13.0±1.0, and 17.1±1.7% respectively. Increases in RDW were observed in patients with iron deficiency and in those receiving iron therapy, as in those with early iron deficiency. Patients with chronic disease, acute hemorrhage, and aplastic anemia with no transfusion in the previous four months had normal RDW. In contrast patients with immune hemolytic anemia, hereditary spherocytosis mechanical hemolytic anemia, acute leukemia, chronic myelocytic leukemia, and chronic hepatobiliary disease had high RDW, as did those with solid tumor and malignant lymphoma during chemotherapy. Among patients with acute leukemia during chemotherapy, RDW was more increased if accompained by macrocytosis than by normocytosis. It was found that the RDW was proportional to the % reticulocyte. We tried to classify anemia, based on RDW and mean corpuscular volume (MCV) and to guide the diagnosis from the peripheral blood analysis in pediatric patients. The distinction of iron deficiency anemia from the anemia of chronic disease and the detection of early iron deficiency was improved. A change in RDW according to storage time at room temperature was not observed. From this study, RDW could be used as a sensitive parameter of red cell anisocytosis. Thus we recommend the use of these new variables the initial classification of anemia in pediatric patients.