The availability of automated blood cell analyzers that provide an index of red blood cell distribution width (RDW) has lead to new approaches to patients with anemia. While the emergency physician is primarily responsible for the detection of patients with anemia, the inclusion of the RDW in the complete blood count has made diagnosing certain anemias easier, especially those that are microcytic. The derivation of the RDW and its clinical application to emergency physicians is discussed and a categorization of anemias based on the mean corpuscular volume (MCV) and RDW is included.