To evaluate the usefulness of haemoglobin A1c (HBA1C) determinations in the diagnosis of diabetes mellitus, the outcome of a standard oral glucose tolerance test (OGTT) and HbA1c values were compared in 178 consecutive subjects referred for a diagnostic OGTT. The subjects were mainly in age groups in which non-symptomatic diabetes mellitus predominates. The prevalences of elevated fasting plasma glucose and abnormal OGTT were found to increase with increasing age. One third of the total series had impaired or diabetic OGTT. In the group with normal OGTT, 16% had fasting plasma glucose concentrations above the upper normal limit, but the range of HbA1c values did not differ from that of healthy controls. In the group with impaired OGTTs (n = 20), elevated HbA1c values were found only in subjects with elevated fasting plasma glucose and the highest two-hour OGTT values (n = 4). In the group with diabetic OGTT, 86-94% had HbA1c values above the upper normal limit, and HbA1c was elevated in all subjects with a fasting plasma glucose higher than 10.5 mmol/l or a 2-hour OGTT glucose value above 14.4 mmol/l. Using the results of the OGTT as the true diagnosis, HbA1c had a diagnostic specificity of 0.78 and sensitivity of 0.96. Only subjects with a substantially reduced glucose tolerance may be diagnosed by HbA1c determinations.