We investigated the presence of autoantibodies (aAbs) directed against the parathyroid gland in 17 patients with spontaneous isolated acquired hypoparathyroidism. Fourteen patients with acquired hypoparathyroidism (AH) associated with type I or II autoimmune polyendocrinopathy syndrome were also tested in comparison with a control group of 68 subjects without AH, including patients with other autoimmune diseases and healthy blood donors. aAbs against parathyroid tissue were screened using an indirect immunofluorescence technique on primate parathyroid tissue and human parathyroid adenoma. aAbs against the calcium-sensing receptor (CaSR) were analyzed using an immunoblotting assay with the recombinant extracellular domain of the human CaSR as antigen. Seven of the 31 patients with AH were positive for CaSR aAbs. Five of the positive sera were obtained from the group with isolated AH. The two other positive sera were from patients with autoimmune polyendocrinopathy syndrome. The sensitivity of the immunoblotting technique was higher than that of both the radioimmunological test using the extracellular domain of the CaSR and the indirect immunofluorescence technique. There were no positive sera in the control group. In conclusion, using an immunoblotting assay, we demonstrate the presence of CaSR aAbs in about one third of the patients with isolated AH, pointing out the value of detecting such aAbs to assess the autoimmune origin of the disease.