INTRODUCTION: Both diabetes mellitus (DM) and iron deficiency anemia (IDA) are prevalent in every area of the world, and so, the possibility of these two diseases co-existing is also very high. It is our belief that clinical results of any correlation between iron status of the body and glycosylated haemoglobin (HbA1c) would be beneficial to many patients, therefore in this study, the effect of IDA on HbA1c was investigated. MATERIALS - METHODS: A total of 146 patients with DM and IDA were evaluated prospectively. While the patients were administered 270 mg/day of ferrous sulphate (80 mg elemental iron) orally for three months for the treatment of IDA, no interventions were made for the treatment of DM. Patient levels of hemoglobin (Hb), hematocrit, red blood cells (RBC), mean corpuscular volume (MCV), platelet, white blood cells (WBC), serum iron, serum iron binding capacity (SIBC), ferritin, fasting plasma glucose (FPG), HbA1c, body mass index (BMI), C-reactive protein (CRP) values were measured at baseline and at the third month of treatment with iron, and were compared. RESULTS: The median age of our patients was 45 (40-50) and median duration of diabetes was 3 years (1,75-5). While the baseline median Hb was 10.4 (mg/dL) (9.5-11.1), MCV was 74 (fL) (70.8-77), ferritin was 4 (ug/L) (3-6) at three months, Hb was measured at 12.6 (mg/dL) (12.1-13.2), MCV was measured at 82 (fL) (80-86), ferritin was measured at 15 (ug/L) (9-21.2) and was significantly higher compared to baseline values (p < 0.001). The baseline median HBA1c of patients was 7.09 ±â€¯0.51 (%) and three month HBA1c was 6.69 ±â€¯0.53 (%), which was significantly lower than when comparing baseline values with values at third month (p < 0.001). Baseline and three month values for FPG were 118 (mg/dL) (108-132) and 116 (mg/dL) (106-125) respectively, and there was no significant difference (p:0.07). A 2.2 mg/dL (1.5-3.5) increase in median Hb level accompanied a 0.4 % (0.2-0.6) decrease in median HbA1c levels (Spearman rho = -0.362; p < 0.001). CONCLUSION: Our study has shown conclusivly that IDA is related to increased HbA1c concentrations and HbA1c decreases significantly following treatment with iron. IDA should be considered before making any decisions regarding diagnosis or treatment according to HbA1c.