Background: Hyponatremia occurring as a result of the syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a common and potentially lethal complication in critically ill patients. Urea, by inducing renal water excretion and promoting sodium (Na) retention, has been well described as a treatment for chronic SIADH. However, there are limited data on its use for the treatment of SIADH as encountered in patients admitted to the intensive care unit (ICU). We assessed the effects of urea administration for treatment of SIADH in ICU patients. Methods: Data from ICU patients treated with urea for SIADH between January 2000 and August 2010 were reviewed. The time courses of Na and urea concentrations were analyzed by variance analysis (ANOVA). Results: Records from 24 patients were analyzed. The most common etiology of SIADH was neurological (18 patients). Before urea administration, the mean serum Na concentration was 124.8 ± 5.9 mEq/l. There was a significant increase in serum Na from the second day of treatment (131.4 ± 3.5 mEq/l, p < 0.001) and a normalization of mean serum Na by the fourth day (136.2 ± 4.1 mEq/l, p < 0.001). The mean serum urea concentration also increased (from 29.8 ± 11.1 mg/dl before urea to 57.6 ± 24.0 mg/dl on the first day of treatment, p < 0.001). Conclusions: Urea administration appears useful for the treatment of SIADH-associated hyponatremia in critically ill patients. Prospective randomized controlled studies are needed to confirm these results.