Urea, the toxic end product of protein catabolism, is elevated in end-stage renal disease (ESRD), although it is unclear whether or how it contributes to disease. Urea can promote the carbamylation of proteins on multiple lysine side chains, including human albumin, which has a predominant carbamylation site on Lys549. The proportion of serum albumin carbamylated on Lys549 (%C-Alb) correlated with time-averaged blood urea concentrations and was twice as high in ESRD patients than in non-uremic subjects (0.90% versus 0.42%). Baseline %C-Alb was higher in ESRD subjects who diedwithin 1 year than in thosewho survived longer than 1 year (1.01% versus 0.77%) and was associatedwith an increased risk of death within 1 year (hazard ratio, 3.76). These findings were validated in an independent cohort of diabetic ESRD subjects (hazard ratio, 3.73). Decreased concentrations of serum amino acids correlatedwith higher%CAlb in ESRD patients, and mice with diet-induced amino acid deficiencies exhibited greater susceptibility to albumin carbamylation than did chow-fed mice. In vitro studies showed that amino acids such as cysteine, histidine, arginine, and lysine, as well as other nucleophiles such as taurine, inhibited cyanate-induced C-Alb formation at physiologic pH and temperature. Together, these results suggest that chronically elevated urea promotes carbamylation of proteins in ESRD and that serum amino acid concentrations may modulate this protein modification. In summary, we have identified serum %C-Alb as a risk factor formortality in patients with ESRD and propose that this risk factor may bemodifiable with supplemental amino acid therapy.