The molecular basis of autosomal dominant neurohypophyseal diabetes insipidus, a hereditary deficiency of vasopressin, was determined by nucleotide sequence analysis of the arginine vasopressin-neurophysin-II gene. A C-->T mutation at nucleotide 1761 was detected in one allele of this gene in each affected individual in three generations of one family. This mutant gene encodes a normal arginine vasopressin peptide, but predicts a substitution of leucine for proline at amino acid 24 of neurophysin-II, the arginine vasopressin carrier protein. This mutation was not detected 50 control individuals, thus demonstrating that it is not a common silent genetic polymorphism. The disease arose in the second generation of the studied family, and the chromosome 20 carrying this new mutation was identified by polymorphic CA microsatellite haplotype analysis. The first affected individual inherited this chromosome segment from her mother, who had neither the disease nor this mutation in her somatic cell DNA. Third generation individuals who subsequently inherited this mutation were affected. These data demonstrate that this amino acid substitution in neurophysin-II causes the disease. Two possibilities to explain the mechanism by which clinical deficiency of arginine vasopressin develops even in the presence of one normal arginine vasopressin-neurophysin-II allele are discussed.