The status of the arginine vasopressin-neurophysin-II (AVP-NPII) gene was studied in three families with autosomal dominant neurohypophyseal diabetes insipidus (AD-NDI). Restriction fragments of genomic DNA containing AVP-NPII sequences from affected individuals were not detectably different in size from those of normal controls. Thus, these individuals with ADNDI do not have apparent large deletions, insertions, or rearrangements of an AVP-NPII allele. Four restriction fragment length polymorphisms were detected with a probe for the adjacent gene on chromosome 20, oxytocin-neurophysin-I (OT-NPI). Linkage studies in these three families between the restriction fragment length polymorphism haplotypes and ADNDI phenotype strongly suggest cosegregation. This indicates that the genetic locus for ADNDI maps within or near the AVP-NPII locus and suggests that a defective AVP-NPII allele may be the basis of ADNDI.