In a family with renal diabetes insipidus, members of four generations were studied; the history of five generations was available. In this kindred the disease appeared to be transmitted as a sex-linked dominant gene attached to the X chromosome, with variable penetrance in the female heterozygous carriers. Renal diabetes insipidus was present in three male children and two men; six additional males died of unknown causes in infancy or early childhood. In males the medical problem is early recognition to avert death or serious brain injury secondary to dehydration. Of five female heterozygous carriers studied, three demonstrated persistently normal urinary concentration following thirsting. Two evidenced marked diminution in renal concentrating ability and could be considered to have mild renal diabetes insipidus. History suggested that one other female member of the kindred had manifest disease. In the female the medical problems are diagnosis in the occasional female with manifest disease and detection of the heterozygous carrier prior to the birth of her child. In this kindred we found the urinary concentration test a reliable detector of the female heterozygous carrier only if the results were abnormally low.