More detailed examinations of the pituitary gland in central diabetes insipidus (CDI) has been possible with magnetic resonance imaging (MRI). Compared to the high signal findings in the posterior lobes of normal subjects, the absence of such signal in adults and children with primary CDI has been reported. Familial CDI is a hereditary form of primary CDI with a variety of clinical expressions among affected individuals which is said to be related to varying degrees of an arginine vasopressin (AVP) deficiency. However, the structural correlates have yet to be documented by MRI technique. This report describes the pituitary gland MRI findings (using a superconducting magnet; 3 mm-slice thickness; spinecho, repetition time 400 ms, echo time 25 ms) in five patients in one family with familial CDI. A signal of high intensity was detected in the posterior part of the pituitary gland in 2 patients but not in 3 others. We concluded that MRI of the posterior pituitary lobe in familial CDI seem to vary in members of the same family.