Résumé : Background: Treatment options for chronic SIADH include water restriction (WR) and urea. The usefulness of urine osmolality to guide the choice of the treatment option is not clearly defined. We hypothesized that urine osmolality can indicate whether treatment with mild water restriction alone could be successful. Methods: Retrospective Review of clinical and biochemical (blood and urine) data of patients with chronic SIADH treated for at least one year with mild WR (1.5–2 l/day) either with or without urea. Results: Twenty nine patients were included. Nine patients were treated by mild WR. Mean serum sodium (SNa) and mean Uosm were 129 ± 2 mEq/l and 274 ± 78 mOsm/kgH2O respectively before WR, and increased to 138.5 ± 3 mEq/l and 505 ± 87 mOsm/kgH2O (P < 0.001). Eight patients were treated with mild WR and 15 g urea daily, the SNa and Uosm before treatment were 127.5 ± 3 mEq/l and 340 ± 100 mOsm/kgH2O respectively and increased to 136.5 ± 1 mEq/l and 490 ± 151 mOsm/kgH2O (P < 0.001). Four of the eight patients had a permanent low solute intake which contributed to hyponatremia. Twelve patients needed 30 g urea daily combined with mild WR. The SNa and Uosm were respectively 126 ± 2 mEq/l and 595 ± 176 mOsm/kgH2O and increased to 136.5 ± 2 mEq/l and 698 ± 157 mOsm/kgH2O (P < 0.05). Uosm increased in most of the treated patients. Conclusions: About 30% of patients could be treated by moderate WR alone. All these patients presented an initial urine osmolality lower than 400 mOsm/kgH2O.