Red cell distribution width (RDW) is a new parameter, measured electronically on the Coulter Counter Model S-Plus. This new parameter measures the degree of anisocytosis, as compared to the average cell size calculated by the MCV parameter. In this study, comparison of RDW values between patients with anemia caused by sickle cell disease and patients with anemia of other causes were significantly different in an ANCOVA statistical analysis (F = 23.0 and p less than 0.001). RDW values greater than 13.0 (Coulter normal range 9.5-11.5) were found in 33 out of 40 patients with sickel cell anemia, whereas only 15 out of 40 patients with anemia of other causes had increased RDW. The RDW value was the only parameter that discriminated between the two groups--MCV and hemoglobin showing no significant difference.