The major form of glycohemoglobin is hemoglobin A1c (HbA1c). The HbA1c fraction is abnormally elevated in chronic hyperglycemic diabetic patients and correlates positively with glycemic control. Previous studies suggest that iron deficiency anemia (IDA) affects the levels of HbA1c. The aim of this study was to determine the effect of IDA on HbA1c levels in nondiabetic patients. The population studied consisted of 50 patients (30 women, 20 men, mean age 35.7 +/- 11.9 years) with IDA and 50 healthy subjects that were matched for age and sex. Patients who had glucose tolerance abnormalities (impaired glucose tolerance or diabetes mellitus), hemoglobinopathies, hemolytic anemia, chronic alcohol ingestion and chronic renal failure were excluded from the study. Hematologic investigations, fasting and postprandial glucose and HbA1c levels were measured in all subjects before iron therapy. All patients with IDA were treated with iron 100 mg/day for 3 months. We repeated the laboratory investigation after iron therapy. Before iron treatment, the mean HbA1c (7.4 +/- 0.8%) level in patients with IDA was higher than in a healthy group (5.9% +/- 0.5) (p < 0.001). In patients with IDA, HbA1c decreased significantly after iron treatment from a mean of 7.4% +/- 0.8 to 6.2% +/- 0.6 (p < 0.001). Iron deficiency must be corrected before any diagnostic or therapeutic decision is made based on HbA1c.