In 35 children with long-standing diabetes mellitus, a significant correlation was found between the hemoglobin A1c (HbA1c)--and the 24-hour urinary glucose excretion. By contrast, 11 newly diagnosed diabetic children had grossly elevated HbA1c-concentrations, but no correlations could be established between the levels of HbA1c and the duration of symptoms, blood glucose, glycosuria, ketonuria and the acid--base status. However, HbA1c and C-peptide were significantly correlated. The elevated HbA1c-concentrations decreased towards normal in all of these 11 children after 2--3 months following adequate therapy. The results suggest that the determination of HbA1c may serve as a valuable metabolic control index in children with long-standing diabetes mellitus, but adds little information in newly diagnosed patients. For the individual diabetic child during the early treatment period, HbA1c may be the index of choice for adequacy of metabolic control.