The rare case of a patient with SIADH following pituitary adenoma apoplexy is reported. Since apoplexy did not exert any mass effect on surrounding structures, the patient was treated conservatively and the anterior pituitary gland insufficiency has been substituted adequately. Seven days after the apoplexy the patient again showed low serum–Na+ levels despite cortisol substitution. Diagnosis of SIADH was made. It is essential to be aware of this rare syndrome in patients with pituitary adenoma apoplexy.