—We read with interest the article by Hansen et al in the January Archives regarding the failure by the nonhematologist to recognize and diagnose thalassemia syndromes. We believe that the diagnostic approach to microcytosis has been considerably improved with the use of the RDW, the utility of which we have recently reviewed1 to heighten the awareness among nonhematologists. The availability of automated blood cell analyzers that calculate the RDW has led to a new approach in the classification of anemia.2 The RDW, which reflects heterogeneity of red blood cell size, differentiates those anemias with heterogeneous populations of red blood cells from those with homogeneous populations. Thus, in microcytic anemia due to iron deficiency, the RDW is characteristically high, indicating the presence of marked anisocytosis. A normal RDW promptly identifies heterozygous thalassemia because of the homogeneity of the red blood cell population. In cases in which iron deficiency and