In autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy, hypoparathyroidism (HP) is the most common endocrine component. It occurs in most (but not all) patients. Determinants of its occurrence are unknown, and there is no proof for its autoimmune nature. Recently, the Ca(2+)-sensing receptor (CaSR) was reported to be an autoantigen in HP. With our group of 90 patients, we aimed at identifying the determinants and pathomechanism of HP. For the determinants, we evaluated gender and the HLA class II. For the pathomechanism, we searched for parathyroid autoantibodies, including antibodies against CaSR and PTH. Also, we studied whether AIRE is expressed in the human parathyroid, because its absence could be a pathogenetic factor. We found a clear gender linkage with lower and later incidence in males. Of the 14 patients who had escaped HP, 13 were males. This was associated with adrenal failure, which was the first or only endocrinopathy in 47% of males vs. 7% of females. In contrast, we found no linkage to the HLA class II. By immunofluorescence, 19% of the patients had antibodies to parathyroid epithelia. By immunoblotting, these recognized several parathyroid proteins. No antibodies were observed against the CaSR or PTH. By RT-PCR, AIRE mRNA was not found in the parathyroid.