Autoantibodies to vasopressin-secreting cells of human hypothalamus were detected by means of indirect immunofluorescence (IFL) in 13 patients with diabetes insipidus (DI). 11 of 30 patients (36 . 7%) with "idiopathic" and 2 of 32 (6 . 3%) with symptomatic DI were positive, and 139 control patients were negative. The specificity of the reaction vasopressin cells was demonstrated with a 4-layer double-fluorochrome IFL test in which the second sandwich consisted of rabbit antivasopressin or anti-oxytocin counterstained with rhodaminated anti-rabbit immunoglobulin. 5 patients had also antibodies to oxytocin-producing cells. The antibodies reacted with cytoplasmic components distinct from the hormone; they were of IgG, IgA, or IgM class or a combination of these classes, and half of them fixed complement. Maximum titres were 1:32, and the antibodies could not be absorbed out by incubation with vasopressin, oxytocin, neurophysin I, or neurophysin II. Some sera stained as yet unidentified small cells in the hypothalamus. This report suggests that autoimmunity extends to the hypothalamus. Vasopressin-cell antibodies may prove to be useful markers for the diagnosis of an autoimmune variant of diabetes insipidus.