To evaluate reticulocyte hemoglobin (RET-Hb) vis-à-vis immature reticulocyte fraction (IRF) as an earliest indicator of response to iron therapy in iron deficiency anemia (IDA), by assessing change in RET-He and IRF at 48 hours after initiation of intravenous iron therapy.A hospital based interventional, analytic study was conducted among 144 patients (age group 15-65 years) with newly diagnosed and untreated IDA admitted in medicine ward and not suffering from any inflammatory disorders (excluded by C-reactive protein). Patient having other forms of anemia/hemoglobinopathies/ malignancy, MCV >80 fL and pregnant female were excluded. All patients were subjected to automated CBC, RET-He, iron studies and iron staining of bone marrow aspirates. Then intravenous iron sucrose was given along with oral antioxidants. After 48 hours, CBC, RET-He and IRF were repeated for each patient.Total 144 patients were included. Of these, 42 patients were excluded due to aparticulate bone marrow aspirate. Remaining 102 patients were classified in to Group A (grade 0 and 1- depleted iron stores) and Group B (grade 2 and 3 - functional iron deficiency). RET-He and IRF increased significantly at 48 hours after initiation of intravenous iron therapy (post therapy) as compared to baseline (pre therapy) in both the two groups as well when all patients were considered together. Post therapy, the mean increase in RET-He was significantly smaller in magnitude in group B than in group A. The increase in IRF was not significantly different between the two groups.RET-Hb, a real time indicator of iron supply (hemoglobinization) to the developing RBC's, is the earliest marker of response to iron therapy as compared to IRF (representative of reticulocyte count).