We describe magnetic resonance imaging (MRI) findings of the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). A 48 year-old man had been involved in a traffic accident and was admitted to our hospital. He was diagnosed as having a slight cerebral contusion and bone fractures in his bilateral lower extremities. His head injury was treated conservatively and his course was uneventful. Eight days after admission his consciousness deteriorated although the computed tomography was markedly improved. His laboratory data showed hyponatremia associated with continued urinary sodium loss and persistent secretion of vasopressin with markedly low plasma osmolality. The patient was diagnosed with SIADH and was treated with fluid restriction, sodium infusion and demeclocycline. T1-weighted MRI of his pituitary gland showed the absence of the high intensity signal of the posterior lobe. The high intensity signal appeared narrowly when his symptoms and hyponatermia improved, and disappeared when his symptoms and hyponatremia deteriorated. The high intensity signal observed with T1-weighted MRI of the normal posterior pituitary reflects an intact and functional neurohypophyseal system. This case suggested that neurosecretory vesicles were decreased despite high plasma vasopressin levels in SIADH patients.