The effect of hypoglycemia--caused by hyperinsulinism in insulinoma patients and in diabetic patients with frequent episodes of hypoglycemia--on glycated hemoglobin was studied. The amount of sugar bound to total hemoglobin in hypoglycemia samples was found to be significantly lower than in those which were normal or hyperglycemic. The amount of total HbA1 fraction, as determined by the mini-column method, was significantly higher than expected on the basis of the corresponding values for total glycated hemoglobin. Evidence is presented to show that this is due to the formation of a hitherto unrecognized HbA1 constituent(s) denoted here as HbA1x.