Glycosylated hemoglobin (HbA1c) is a marker of evaluation of long-term glycemic control in diabetic patients and predicts risks for the development and/or progression of diabetic complications. Glycosylation process depends on the exposure to glucose, so on the half-life of erythrocyte. It was demonstrated, however that metabolic control concerning the last 90-120 days had only a 10% effect on the result of HbAlc; mean blood glucose of the last 30 days contributes for 50% of HbA1c value. Blood glucose value in the afternoon and in the evening better correlate with HbA1c levels if compared with blood glucose values in the morning. It is important to know that there may be, in the evaluation of HbAlc, interference in the dosage, due to a condition of uremia, hyperlipemia, bad conservation and hemolysis of the sample, increase of leucocytes and presence of anomalous hemoglobins. Moreover the use of different methods, the lack of a common calibration concerning the same methods and the variability of instrumentation do not make reproducible results yet, in different laboratories.