HbA1 was measured by a rapid method in a routine hospital laboratory. The normal range of 5.0--8.5% compares favourably with other studies. Patients with uncontrolled diabetes had levels of HbA1 of up to 20.5% while well controlled patients had levels of HbA1 in the normal range. The level of HbA1 is relatively stable in normal subjects and does not correlate with age, sex or weight. There was a correlation (p less than 0.001) with a 1-2 hour post-prandial, post-insulin blood sugar in insulin dependent diabetics. There was a relationship between type of therapy and HbA1 level at presentation. From the study of two ketoacidotic patients, it is apparent that the level of HbA1 decreases about six weeks after the decrease in blood sugar. It is concluded that the measurement of HbA1 is a useful adjunct in the assessment of diabetic control.