Iron deficiency anaemia (IDA) and beta-thalassaemia are the most common causes of microcytic anaemia. Some indices have been defined to quickly discriminate this diseases based on red cell parameters obtained from automated blood cell analyzers, and can be effective for use as a preliminary screening tool to allow the reflex HbA(2) analysis, when a proper cut-off is chosen. Advia 2120 (Siemens Medical Solutions Diagnostics) directly measures volume and haemoglobin concentration of individual red cells, and quantifies the percentage of microcytic, normocytic, macrocytic, hypochromic, normochromic and hyperchromic red cells. Because of the inverse behaviour of the % microcytic and % hypochromic red cells in beta-thalassaemia trait and in IDA the ratio between these two values was computed and its discriminant efficiency assessed. The aim of the study was to assess the predictive value of the new index % microcytic/% hypochromic ratio in the differential diagnosis of beta-thalassaemia compared with Mentzer index, currently used in our Laboratory. Sensitivity, specificity and total efficiency of both indices were calculated for a set of 110 IDA patients and 150 beta-thalassaemia carriers. Discriminant efficiency was similar for both indices.