The posterior lobe of the pituitary gland was evaluated by 1.5 T magnetic resonance (MR) in five cases of diabetes insipidus (DI), including one primary (idiopathic) and four secondary DI cases due to germinomas (two), teratoma (one), and histiocytosis X (one). The normal posterior lobe displays high signal indistinguishable from fatty tissue on T1-weighted images (T1WI) (short T1 value). In all five DI cases the normal high signal of the posterior lobe was not detected in the pituitary fossa on T1WI. Hence, because of this characteristic finding, MR may greatly assist in the diagnosis of DI. We may speculate that the short T1 value of the posterior lobe is closely related to its functional integrity and may be due to the neurosecretory materials in the axons of the hypothalamohypophyseal tract.