The level of the minor glycosylated haemoglobins (HbA1) in ambulatory diabetic patients correlated closely with their physicians' ratings of the degree of control and their fasting plasma-glucose levels. In patients admitted to hospital for more detailed study, HbA1 correlated significantly with the mean fasting glucose, mean daily glucose, and highest daily glucose values. HbA1 measurement is a simple, rapid, and objective procedure to assess diabetic control and may serve both as a screening test for uncontrolled diabetes and as an indicator of the efficacy of various therapeutic regimens.