For each family, the entire vasopressin V2 gene of at least 1 affected male, and where possible at least 1 obligate carrier and 1 unaffected brother of a patient is sequenced. In addition, a detailed Xq28 haplotype analysis is done to identify the origin of de novo mutations. If no mutation is found and the disorder is not transmitted in an X-linked mode, both alleles of the gene that codes for aquaporin-II are also sequenced. DNA is collected by mail from as many kindred as possible who do not participate in the clinical studies.