A 4-year-old boy was treated with oxybutinine and desmopressine because of bladder instability associated with secondary enuresis. He was admitted with obnubilation, vomiting and experienced two seizure episodes concomitantly with hyponatremia and hypoosmolality. The child healed promptly under water restriction and intravenous administration of sodium chloride. This case report suggests that desmopressine may be responsible for severe side-effects. This drug should not be widely used and its indications should be restricted to patients with proven antidiuretic hormone secretion abnormalities.