Colorimetric determinations of glycosylated haemoglobin by the thiobarbituric acid method were carried out in normal adults (n = 142), newly born infants of healthy mothers (n = 81), and in a group of patients with diabetes mellitus (n = 124). The glycosylated haemoglobin level in normal adult males was 4.9%, which is close to that reported for other populations. No correlation was found between age and the levels of glycosylated haemoglobin in males over 10 years old. However, the mean value for glycosylated haemoglobin in cord blood was 4.1%, significantly different from that of adults. The range values for glycosylated haemoglobin in diabetic patients was 7-19%. The mean value for glycosylated haemoglobin was 10.9%, similar to that established in other diabetic populations. Results of colorimetric determinations of glycosylated haemoglobin in the Saudi population compare well with other ethnic groups and, therefore, suggest no ethnic differences in glycosylated haemoglobin levels.