The purpose of this study was to evaluate red cell distribution width (RDW) in the differential diagnosis of patients with well defined iron deficiency (ID), anemia of chronic disease (ACD), and thalassemia trait. Mean RDW in 100 "normals" was 13.1%. Elevated RDW (greater than 15.0%) was observed in 90% of ID (N = 39), 32% of ACD(N = 79), 66% of beta-thalassemia trait (N = 100), 53% of alpha-thalassemia trait men (N = 127), 34% of alpha-thalassemia trait women (N = 116), 3% of sickle cell trait (N = 101), and 88% of sickling disorders (N = 32). The Bessman classification by mean corpuscular volume and RDW was of limited value in separating ID from thalassemia trait but was improved by adopting 16.0% as the upper limit of normal for RDW. Although many attempts have been made to discriminate between ID and thalassemia trait on the basis of red cell values from a cell counter, none (including RDW) is sufficiently specific or sensitive to obviate the need for confirmation of the diagnosis by standard laboratory methods.