Red cell distribution width (RDW) was studied in adults carrying delta-beta thalassemia traits (delta beta -TT) who were 20-40 years of age (n = 29), beta thalassemia traits (beta -TT) with an age range of 18-60 years (n = 49), iron deficiency anemia (IDA) in individuals aged 1-18 years (n = 27), and In controls with an age range of 20-40 years (n = 20). Although red blood cell count, MCV, and MCH values showed no statistically significant differences between delta beta -TT and beta -TT, the mean RDW value was significantly higher in delta beta -TT (20.14 +/- 1.21) compared to beta -TT (14.88 +/- 1.77) (P < 0.001). No difference was observed between the means of RDW in delta beta -TT and IDA (18.00 +/- 1.94) (P > 0.05). A significant rise in RDW in IDA 5-7 days after initiation of iron therapy (P = 0.00) which was continued to rise up to the 4(th) week of therapy was suggested as an important tool in differentiation of IDA from delta beta -TT. These observations could be kept in mind in the differential diagnosis of delta beta -TT from beta -TT and IDA by determining the red blood cell count, red cell indices, and RDW only. (C) 2002 Wiley-Liss, Inc.