A review of human parathyroid hormone measurements is presented. Although parathyroid hormone was among the first hormones to be measured with the radioimmunoassay technique in 1963, there are even now still many pitfalls in both the technical aspects and clinical applicability of this assay. The main reasons for these difficulties are presented: firstly the complex intra- and extraglandular metabolism of parathyroid hormone results in the heterogeneity of the hormone in serum; secondly, the scarceness of pure human parathyroid hormone delays the development of a complete homologous human assay. The existing assays however have, despite their limitations, enormously contributed to our knowledge of parathyroid hormone secretion in both the normal state and in disease. The clinical applicability of these assays is reviewed and future technical improvements are suggested.