The diagnostic value of serum ferritin measurements in discriminating iron-deficiency anemia from thalassemia trait has been studied. In contrast to serum iron, percent transferrin saturation and total iron-binding capacity, where a high degree of overlap existed between the two groups, a clear-cut difference in serum ferritin levels was found between iron deficiency and thalassemia trait. The best separation of iron deficiency, thalassemia and normal controls was given by the combination of mean corpuscular volume and serum ferritin. Although definitive diagnosis of beta-thalassemia trait requires the demonstration of abnormal Hb A2 levels or beta-chain synthesis, serum ferritin is a useful screening test for the initial diagnosis of thalassemia trait. Because of the very small amounts of serum required for the measurement of ferritin, it is particularly suitable for surveying populations with a high prevalence of hypochromic-microcytic anemias.