The syndrome of inappropriate secretion of antidiuretic hormone (SIADH) is a frequent complication of severe head trauma. Fluid restriction is treatment choice of SIADH in patients with traumatic brain injury (TBI), but fluid restriction is limited because they need sufficient calories. We described a patient who, on the four months after a head injury, presented with deterioration of consciousness, which coincided with the development of the SIADH, and which rapidly reversed with the correction of the hyponatremia by demeclocycline with minimal fluid restriction. We suggest that SIADH should be included in the differential diagnosis of deterioration of consciousness during the recovery period of the patients suffering from head injury because unexpected clinical deterioration may often have a reversible cause. Also, demeclocycline will be useful, which allows for increased fluid liberalization and for provision of adequate calories, in the treatment of the SIADH in patients with TBI. (J Korean Acad Rehab Med 2003; 27: 438-441)