Background: Erythrocyte ferritin (EF) reflects the balance between iron supply and its utilization for hemoglobin synthesis. This balance is altered in microcytosis. We aimed to evaluate the diagnostic value of both EF and the ratio (FRR) plasma ferritin (PF)/EF in these disorders.Methods: A total of 231 subjects participated in the study. Samples from 93 adult patients with different causes of microcytosis, 57 healthy subjects and 81 full term newborns were analyzed to determine EF and PF concentrations and other hematological parameters. Results:In patients with iron deficiency, and in contrast to PF, EF decreased only in the presence of anemia and in direct correlation with the degree of microcytosis (Pearson´s p<0.001). EF values for thalassemia patients were higher than those observed in controls (p < 10e-5), whilst PF concentrations were similar between these groups .This EF increase was more marked in the delta-beta thalassemia group (p<0.05). Finally, FRR was much higher in patients with anemia of inflammation than in those with thalassemia (p<10e-5), thus helping to discriminate between these disorders.Conclusions: EF and FRR are tools that may be useful in the diagnosis of the main causes of microcytosis.