New automated blood cell analyzers provide an index of red cell volume distribution width (RDW) or heterogeneity and a histogram display of red cell volume distribution. We have developed a classification of red cell disorders, based on mean corpuscular volume (MCV) or red cell size, heterogeneity, and histograms, to guide diagnosis from the peripheral blood analysis. The distinction of iron deficiency anemia from heterozygous thalassemia or the anemia of chronic disease and the detection of early iron and folate deficiency is improved. Red cell volume distribution histograms identify red cell fragmentation or agglutination, dimorphic populations, and artifactual counting of lymphocytes as red cells. We recommend the use of these new variables in the initial classification of anemia by the practicing physician.