The detection of carbamylated hemoglobin (CarHb) is known to be useful in determination of the chronicity of uremia. However, the time course of the in vivo reaction between isocyanic acid and terminal valine residues of the hemoglobin chain is not clearly defined. To assess the temporal relationship and reversibility of carbamylation, we prospectively measured CarHb as micrograms of valine hydantoin per gram of hemoglobin (microg VH/g Hb) by high-performance liquid chromatography in 37 patients with acute renal failure (ARF), 53 patients with chronic renal failure (CRF), and six patients with successful kidney transplant. Patients with ARF had a lower median CarHb concentration (53.2 microg VH/g Hb; range, 24.6 to 97.1 microg VH/g Hb) than those with CRF (115.0 microg VH/g Hb; range, 34.6 to 286.5 microg VH/g Hb; P < 0.01), but had a higher value (53.2 microg VH/g Hb; range, 24.6 to 97.1 microg VH/g Hb) than 31 normal controls (36.6 microg VH/g Hb; range, 19.9 to 62.9 microg VH/g Hb; P < 0.05). In patients with ARF, the CarHb concentration positively correlated with the number of days of illness (r = 0.74; P < 0.01). The patients with ARF of 10 or more days' duration had a higher CarHb concentration (68.7 microg VH/g Hb; range, 36.0 to 93.9 microg VH/g Hb) than those with a shorter duration of ARF (33.7 microg VH/g Hb; range, 24.6 to 55.8 microg VH/g Hb; P < 0.01) despite similar blood urea nitrogen and serum creatinine values. However, they had a lower concentration of CarHb (68.7 microg VH/g Hb; range, 36.0 to 93.9 microg VH/g Hb) than CRF patients with comparable serum creatinine values (112.5 microg VH/g Hb; range, 34.6 to 286.5 microg VH/g Hb; P < 0.01). In patients with a kidney transplant, CarHb concentration declined by 19.7% (range, 12.3% to 35.6%) within 2 to 3 weeks after receiving the graft, while the level of hemoglobin increased by 25% (range, 4.0% to 46.6%) during the same period. Therefore, the total blood CarHb (CarHb x hemoglobin concentration) was not significantly changed. We concluded that the in vivo reaction of carbamylation of hemoglobin progressed during the period of uremia, and there might exist some irreversible preformed CarHb in advanced stages of CRF.