In the syndrome of inappropriate secretion of antidiuretic hormone (SIADH), rapid elevation of serum sodium concentration may be imperative to correct neurological symptoms. Seven patients with hyponatremia secondary to SIADH were treated by oral intake of two to three doses of 30 g of urea over 24 hours or infusion of 80 g of urea as a 30% solution of over six hours, water restriction (500 mL/24 hr), and sodium supplements (120 to 360 mmole/24 hr). Serum sodium concentration increased from 117 +/- 2 to 126 +/- 1.4 mmole/L (mean +/- SEM) after eight hours, to 130 +/- 1.3 mmole/L after 12 hours, and to 134.5 +/- 1.2 mmole/L after 24 hours. The normalization of serum sodium was secondary to osmotic diuresis and to sodium retention induced by urea. Use of urea should be considered when symptomatic hyponatremia in SIADH must be quickly corrected.