A novel red cell discriminant function [MCV2 x RDW/(Hgb x 100)] was compared to six other discriminants in 102 patients with established mild iron deficiency anemia and 33 patients with beta-thalassemia minor. The discriminant incorporates the two key measurements of erythrocyte cell volume distribution, namely the mean (MCV) and standard deviation (RDW), which are known to be helpful for distinguishing between these two frequent causes of microcytic hypochromic anemia. Data used for the learning set to develop the new discriminant were obtained using an electrical impedance automated whole blood analyzer (Coulter S + IV) and were applied as a validation set for six other discriminants. The discriminant was also tested on smaller subsets of the patients groups using data obtained on either an alternate electrical impedance instrument (Sysmex E-5000) or a laser light scattering based system (Technicon H*1). From the comparison it was concluded that use of a discriminant function that incorporates a measurement of red cell volume dispersion results in enhanced accuracy for distinguishing iron deficiency anemia from thalassemia minor.