Hemoglobin A1 concentrations were measured in 230 patients with diabetes mellitus. Diabetic patients controlled by diet alone had significantly lower HbA1 levels compared to the insulin or oral hypoglycemic treated groups. Only 13% of insulin-treated patients had HbA1 levels below 10%, while 26% of the oral agent treated group and 70% of the diet treated group fell into this range. The HbA1 levels correlated with fasting and nonfasting plasma glucose concentrations. However, in certain patients, discrepant results between these two variables were found. Hemoglobin A1 was measured on three or more occasions in 60 patients over a 1-yr period. An improvement was noted in 40%, no change in 38%, and a deterioration in 22%. Hemoglobin A1 measurements have proved to be useful in the follow-up and treatment of diabetic patients.