To assess the relationship between the length of (GT)n repeats in HO-1 gene promoter and heme oxygenase (HO) enzymatic activity in mononuclear cells with iron (Fe) stores in type 2 diabetic mellitus (DM2) patients and metabolic syndrome (MS) subjects, we studied 163 patients with DM2, 185 with MS, and 120 controls subjects. We evaluated iron status (hemoglobin and serum Fe, ferritin, and transferrin receptor), and we determined the length of (GT)n repeats in HO-1 gene promoter by capillary electrophoresis and HO enzymatic activity in mononuclear cells and assessed the relationship between these results and Fe stores. Only 1/163, 6/185, and 7/120 had iron deficiency anemia in DM2 patients, MS subjects, and controls, respectively. No iron overload (ferritin>200 μg/L) was detected in all the subjects studied. DM2 patients had higher iron deposits, total body iron, and heme oxygenase activity (a suggestion of high oxidative stress condition) than MS subjects and controls. In DM2, we found a positive association between serum iron and HO activity. There were no difference in allelic frequency between the three groups; however, among DM2 and MS patients, the frequency of short/medium (SM) genotype of (GT)n repetition was increased and medium/medium (MM) genotype of (GT)n repetition was lower than controls. These results imply that DM2 patients and individuals with MS carrying SM repeats might have higher susceptibility to develop diabetes consequences. This increased susceptibility could be Fe-mediated oxidative stress.