OBJECTIVE We aimed at finding out reliable parameter in the differentiation of iron deficiency anemia (IDA) and beta-thalassemia trait (β-TT) in the adult population subjected to Saudi Arabian Premarital Screening Program. METHODS A total of 620 adults (age range 21-36 years) reported during February 2012 to November 2012. Tests for serum iron and ferritin were carried out in individuals showing low hemoglobin (Hb). All the selected subjects' samples were subjected to blood morphology, comparison of MCV, RBC count. Red Cell Distribution Width (RDW) was noted from the Coulter Report whereas Red Cell Distribution Width Index (RDWI) value was calculated for all the samples. RESULTS A total of one hundred &thirty-five individuals with hypochromic microcytic anemia having normal hemoglobin F and hemoglobin A2 < 3.2% were inducted in the study. Ninety-three were diagnosed having IDA, whereas thirty-two were having βTT. Ten individuals revealed other causes of anemia. The RBC count was higher, and MCV was much lower in βTT as compared to IDA. Both groups were subjected to RDW and RDWI, however, RDWI which showed better sensitivity and specificity for βTT. CONCLUSIONS RDWI is a reliable and useful index for differentiation among IDA and βTT, as compared to RDW.