The red cell distribution width index (RDW) was determined in 103 normal children, 69 iron-deficient (ID) patients, 73 with the thalassaemia trait, and 71 with other haemoglobinopathies. Elevated RDW values were found in anaemic patients, the highest values in ID anaemia, sickle thalassaemia, sickle cell anaemia, and beta-thalassaemia trait in decreasing order. The normal RDW in children was 13.2 +/- 0.9 and an elevated RDW reflects active erythropoiesis. The RDW was elevated and the MCV low in all 69 patients with ID anaemia. The RDW was also elevated in 11/13 children with sickle cell anaemia, in 25/29 patients with sickle cell-thalassaemia, and in all patients with thalassaemia major. The ID anaemia could be differentiated from the thalassaemia trait by the markedly elevated RDW in ID anaemia (mean = 20.7 +/- 3.2) and the mildly elevated RDW (mean 15.4 +/- 1.4) in thalassaemia trait. The RDW index discriminated better than Mentzer's index, discriminant function or the Shine Lal index. Thus the RDW is a good indicator of anisocytosis, and a good screening index especially for ID anaemia and the thalassaemia trait.