Congenital nephrogenic diabetes insipidus (NDI) is an X-linked inherited disorder characterized by renal resistance to the antidiuretic hormonal action of arginine vasopressin. The disease gene has been assigned to the subtelomeric region of the X chromosome long arm by demonstrating close linkage between NDI and several X-chromosomal DNA markers. The finding of closely linked genetic markers is useful in the diagnosis of NDI. Receptor studies in patients have indicated that NDI might be due to the absence or an abnormality of the adenylate cyclase-bound vasopressin type 2 receptor. This assumption was supported by the discovery of functional vasopressin V2 receptor activity in somatic cell hybrid cell lines that carried at least the distal part of the human X chromosome long arm. Definite evidence for a V2 receptor defect being the cause of NDI was found in a recent study demonstrating point mutations in the V2 receptor gene from affected individuals. Direct mutation analysis is now applicable for accurate carrier detection and early (prenatal) diagnosis.