The diagnostic usefulness of red cell distribution width (RDW) in association with usual biochemical and haematological parameters in detection of iron deficiency has been studied in a representative sample population of 384 children aged six months to six years in Reunion. Traditional parameters measured included serum ferritin (Fri), total iron binding capacity (TIBC), serum iron (SI), transferrin saturation (TSat), free erythrocyte protoporphyrin (FEP), mean corpuscular haemoglobin concentration (MCHC), mean corpuscular haemoglobin (MCH), mean corpuscular volume (MCV) and haemoglobin concentration (Hb). RDW is an index of the variation in red cell size (anisocytosis). This recently derived parameter is measured by some models of electronic cell counter. It is not usually used in epidemiological investigations. Of the children studied, 13.6% had Hb less than 11 g/dl. The Pearson correlation coefficients between circulating iron parameters (SI, TSat, TIBC) or iron storage parameters (Fri) and RDW, MCV, MCH and FEP were greater than with Hb. The best correlations were observed for RDW, MCV and MCH with all other parameters. In this study, the upper limit value of RDW was defined as 18% using a Technicon model H-6000 counter. Other iron deficiency criteria were also defined and found to be in agreement with the international reference values for children aged six months to six years; MCV less than 70 fl, MCH less than 22 pg, MCHC less than 32%, FEP greater than 35 micrograms/dl whole blood, SI less than 6 mumols/l, TIBC greater than 85 mumols/l, TSat less than 10% and Fri less than 12 micrograms/l. The combination of sensitivity and specificity was best for RDW and worst for MCHC.(ABSTRACT TRUNCATED AT 250 WORDS)