Red cell distribution width (RDW) on the Coulter S-Plus has many limitations. Normal values (mean +/- 2 SD) were found to be sex dependent: 10.7 +/- 2.4 for males, and 9.8 +/- 1.8 for females. These ranges are broader than the normal range of 10.0 +/- 1.5 given by Coulter. When these broader ranges were applied, none of 29 patients with iron deficiency anemia showed an elevated RDW. Furthermore, a decrease in RDW with room temperature storage was observed and may contribute to erroneous results. The Coulter S-Plus RDW may not be as useful in quantitating anisocytosis as other methods of measuring the dispersion of the red cell volume histogram.