The syndrome of inappropriate antidiuretic hormone secretion (SIADH) developed approximately 7 days after a spontaneous subarachnoid hemorrhage in a 63-year-old woman with an anterior cerebral artery aneurysm. The hyponatremia associated with this syndrome resulted in a deterioration of the patient's clinical condition and focal neurological signs, which simulated the clinical deterioration after spontaneous subarachnoid hemorrhage that is often caused by other intracranial pathological conditions. The focal neurological signs in particular are likely to be interpreted as indicating one of these other conditions. Prompt recognition and treatment of the SIADH resulted in prompt improvement, and we were then able to proceed with the planned craniotomy for the aneurysm. The syndrome and its importance are discussed.