The red cell distribution width (RDW) is an index of the variation in red cells size (anisocytosis). A study was conducted to examine the validity of using RDW in improving classification of microcytic anaemias. A total of 300 blood samples collected from a patient population aged 3 months to 55 years who were referred for haemoglobin electrophoresis were examined at the Aga Khan University Hospital (AKUH). On complete blood count, initially 200 patients (66.6%) were found to have hypochromic microcytic anaemia. Following haemoglobin electrophoresis 41% (82/200) patients were diagnosed to have thalassemia minor and 59% (118/200) had hypochromic microcytic anaemia with either a normal haemoglobin pattern or an abnormal haemoglobin. The mean (+/- SD) RDW estimated in 250 apparently healthy Pakistani population was 14 (+/- 1.5%). Elevated RDW of 23% was present in 94% (110/118) of the patients with hypochromic microcytic anaemia due to other causes, whereas 48% (39/82) of the patients with thalassemia minor had elevated RDW of 16%. Also, of the 82 thalassemia minor, 28 patients had normal haemoglobin level, of which 6 had elevated RDW and of the 54 with low haemoglobin level, 35 had elevated RDW. RDW was normal in 41 patients with thalassemia minor trait. Our results suggest that RDW alone cannot be used as a reliable indicator to distinguish between thalassemia minor and other causes of microcytosis.