The red cell distribution width (RDW), which provides a quantitative measure of heterogeneity of red cells in the peripheral blood, and the mean corpuscular volume (MCV) are part of the routine red cell indices reported by automated blood analyses. This study evaluated 193 pediatric patients with a wide range of erythrocyte disorders and determined the diagnostic utility of the RDW in relation to the MCV. Six different groups of erythrocyte disorders by MCV and RDW values are described: low MCV/normal RDW, low MCV/high RDW, normal MCV/normal RDW, normal MCV/high RDW, high MCV/normal RDW, high MCV/high RDW. This combination established a useful differential diagnosis of erythrocyte disorders. The data provided a baseline against which future studies of infants and children can be compared, though each laboratory has to verify its own normals. It should be cautioned that different electronic counters yield different RDW values, so there have to be qualifications when reporting reference values. The RDW may find its best use as a guide in the differential diagnosis of anemia, rather than as a definitive test per se.