The availability of automated blood cell analyzers that provide an index of red blood cell volume distribution width (RDW) and a mean platelet volume (MPV) has led to new approaches to patients with anemia and thrombocytopenia. The RDW, which measures heterogeneity of the red blood cell population, complements the mean corpuscular volume in the differential diagnosis of anemia based on peripheral blood analysis. The MPV varies inversely but nonlinearly with the platelet count in normal individuals and is of value in assessing platelet production in the thrombocytopenic patient. The clinical applications of the RDW and MPV, which are currently reported on most routine blood cell counts, are discussed.