Glycosylated and carbamylated haemoglobin were determined in patients with uraemia and/or diabetes mellitus. Glycosylated haemoglobin measured by ion-exchange chromatography (HbA1, HbA1c, HbA1a + b) was elevated in non-diabetic uraemic patients, while colorimetrically determined glycosylated haemoglobin was similar to controls. Patients with diabetes mellitus and normal renal function had similar glycosylated haemoglobin concentrations to those with renal failure. Both methods showed an excellent correlation, independent of renal function, in patients with diabetes mellitus. The HbA1c component was more influenced by diabetes and the HbA1a + b component was relatively more dependent on renal function. Carbamylated haemoglobin was detected in all subjects, but was grossly elevated in uraemia. Carbamylated haemoglobin significantly correlated with renal function and chromatographically determined glycosylated haemoglobin. Data from this study strongly suggests that the apparent elevation of chromatographically determined glycosylated haemoglobin in uraemia is due to the increased formation of carbamylated haemoglobin. However, in patients with diabetes mellitus, independent of renal function, both the chromatographic and colorimetric methods of determining glycosylated haemoglobin are equally valuable and reliable. The non-enzymatic formation of carbamylated haemoglobin in uraemia has several similarities to glycosylated haemoglobin in patients with diabetes mellitus. Carbamylated haemoglobin may have a clinical role as a marker of uraemia and may also have a pathophysiological relevance.