In 65 children and adolescents with type I diabetes and in 23 healthy controls, glycosylated hemoglobin (HbA1) was measured by ion exchange chromatography in a specimen of whole blood and in three fractions of erythrocytes of different ages. The red blood cells were separated in a density-gradient medium. Three layers were obtained. The top one consisted of younger and lighter cells, while the heaviest and oldest cells were found in the bottom layer. The separation was evaluated by measuring the potassium concentration, which was significantly different in each fraction (p < 0.001). In all healthy controls the lowest HbA1 value was measured in the youngest and the highest in the oldest erythrocytes. In the diabetic patients, however, the HbA1 levels in individual red cell fractions depended on the degree of metabolic control during the life span of the cells. Significant correlation was found between the HbA1 levels in the youngest erythrocytes and metabolic control in the two-week period before investigation (p < 0.001), between the HbA1 content of the medium-aged cell fraction and control two months before assay (p < 0.001), and between HbA1 levels in the oldest erythrocytes and control three and especially four months prior to investigation (p < 0.001). The results of HbA1 determination in the fraction of the youngest cells which show the highest correlation (-0.75) with the glucosuria two weeks prior to assay indicate to be one of possibilities to get information about metabolic control in that period.