In order to ascertain whether there are abnormalities of nonenzymatic glycosylation in uremia, the levels of nonenzymatically glycosylated hemoglobin (GHb), and total plasmatic glycosylated proteins (PGP) were studied using the thiobarbituric acid (TBA) method, a procedure not interfered with by carbamylation. Total hemoglobin A1 (HbA1) and the A1c fraction were also determined by ion exchange chromatographic methods. Sixty-six end-stage renal disease patients (29 nondiabetic and 8 diabetic uremic patients on conservative treatment, 29 nondiabetic hemodialysis patients) and 56 controls (32 nonuremic diabetic patients and 24 healthy controls) were studied. High levels of GHb and total PGP were found in the nondiabetic uremic group on conservative treatment with all the methods used, but the persistence of high chromatographically determined HbA1 levels in hemodialysis patients contrasts with the results obtained with the other techniques, which showed lower values on hemodialysis. Nondiabetic uremic patients with abnormal oral glucose tolerance curves had significantly higher levels of TBA-determined GHb and PGP. Uremic diabetic patients had the highest glycosylation levels of all the studied groups. We conclude that there is an abnormal nonenzymatic glycosylation of proteins in uremia, independent of carbamylation reactions and partially corrected by hemodialysis.