The concentration of hemoglobin A1c has been found to be elevated in patients with uncontrolled diabetes. HbA1c levels change at a slow rate so that a single measurement is believed to reflect the overall degree of control achieved by a patient for about the previous 2 mo. Using the Trivelli method, we determined HbA1c levels in 81 children with overt insulin-dependent diabetes and in 14 healthy children and young adults who served as controls. Five of the diabetic children with recent onset had sequential HbA1c determinations at 3–6-wk intervals. Each had a rapid decline in HbA1c level to near normal values after the patient was kept essentially euglycemic for about 60 days. Before the HbA1c determination, each diabetic child was given an overall control rating for the previous 2 mo or more based on criteria defined in our clinic. HbA1c values increased progressively as diabetic control declined. There was a statistically significant correlation between the clinical control ratings and HbA1c levels (P < 0.01 between all subgroups). Although there was a positive correlation between duration of diabetes and insulin requirement per kilogram of body weight in all groups combined, there was no significant difference when each subgroup was compared separately. These observations document the validity of our criteria for classifying patients in higher degrees of metabolic control.