Carbamylated hemoglobin (Carb Hb) levels were measured in 16 patients with a documented transient rise in BUN due to prerenal azotemia, in whom BUN levels before and after the episode were normal. They were compared with 13 controls. Carb Hb was expressed as carbamyl valine concentrations, which were significantly higher in the patients (166 μg/g Hb) than in controls (95.3 μg/g Hb, p < 0.01). The mean maximum BUN level in the patients was 51.8 ± 23.9 mg/dl. There was a significant correlation between the product of mean BUN level times the number of days of BUN elevation, and the Carb Hb level (r = 0.5197; p < 0.05). There was no correlation between Carb Hb and either mean BUN level or maximum BUN level. Elevated Carb Hb was seen after a minimum of 4 days’ BUN elevation. Four patients had no elevation of Carb Hb despite elevated BUN levels. Conclusion: Carb Hb may be elevated after a minimum of 4 days’ transient BUN elevation; Carb Hb is not useful in differentiating between mild acute renal failure and prerenal azotemia.