The RBC histogram is an integral part of automated hematology analysis and is now routinely available on all automated cell counters. This histogram and other associated complete blood count (CBC) parameters have been found abnormal in various hematological conditions and may provide major clues in the diagnosis and management of significant red cell disorders. In addition, it is frequently used, along with the peripheral blood film, as an aid in monitoring and interpreting abnormal morphological changes, particularly dimorphic red cell populations. This article discusses some morphological features of dimorphism and the ensuing characteristic changes in their RBC histograms.