In 44 diabetic patients, who within 7 days lowered their average glycemia by 140 to 220 mg % and kept it at this level over a mean of 36 days, HbA1c was measured repeatedly by HPLC (normal range 4.2-5.6% of total hemoglobin). Thereby, the maximum decay rate of HbA1c could be observed in vivo as being 0.1% per day which is consistent with previous theoretical considerations by other authors. Some clinical implications that this finding may have for diabetic patients are being discussed.