To establish a normal range of hemoglobin (Hb) AIc, and ascertain the sensitivity and reproducibility of this test, three groups were studied: patients with suspected diabetes undergoing a standard oral glucose tolerance test, diabetic outpatients, and subjects with normal carbohydrate tolerance studied on two separate occasions. In Group 1, the mean Hb AIc (± SD) in 165 carbohydrate-tolerant subjects was 4.97 ± 0.50% and the normal range was the mean ± 2 standard deviations (4.0 to 6.0%). The mean Hb AIc (± SD) of the 63 carbohydrate-intolerant subjects was 6.45 ± 1.14, significantly greater than the normal subjects. In both subgroups, Hb AIc correlated significantly with the fasting blood glucose but not with the glucose areas of the oral glucose-tolerance test. In Group 2, the Hb AIc values were normal in 39% of those diet treated, 10% of those on oral agents, and none of those on insulin therapy. In Group 3, the mean coefficient of variation of the two Hb AIc values for each subject was 6.86% but was 7.15% for the fasting blood glucose. These findings suggest that Hb AIc is highly reproducible and responsive to minor degrees of abnormality of glucose tolerance and may provide an alternative method for defining carbohydrate tolerance or the degree of blood sugar control.