We developed a novel immunoradiometric assay (IRMA; whole parathyroid hormone [PTH] IRMA) for PTH, which specifically measures biologically active whole PTH(1–84). The assay is based on a solid phase coated with anti-PTH(39–84) antibody, a tracer of125I-labeled antibody with a unique specificity to the first N-terminal amino acid of PTH(1–84), and calibrators of diluted synthetic PTH(1–84). In contrast to the Nichols intact PTH IRMA, this new assay does not detect PTH(7–84) fragments and only detects one immunoreactive peak in chromatographically fractionated patient samples. The assay was shown to have an analytical sensitivity of 1.0 pg/ml with a linear measurement range up to 2300 pg/ml. With this assay, we further identified that the previously described non-(1–84)PTH fragments are aminoterminally truncated with similar hydrophobicity as PTH(7–84), and these PTH fragments are present not only in patients with secondary hyperparathyroidism (2°-HPT) of uremia, but also in patients with primary hyperparathyroidism (1°-HPT) and normal persons. The plasma normal range of the whole PTH(1–84) was 7–36 pg/ml (mean ± SD: 22.7 ± 7.2 pg/ml, n = 135), whereas over 93.9% (155/165) of patients with 1°-HPT had whole PTH(1–84) values above the normal cut-off. The percentage of biologically active whole PTH(1–84) (pB%) in the pool of total immunoreactive “intact” PTH is higher in the normal population (median: 67.3%; SD: 15.8%; n = 56) than in uremic patients (median:53.8%; SD: 15.5%; n = 318; p < 0.001), although the whole PTH(1–84) values from uremic patients displayed a more significant heterogeneous distribution when compared with that of 1°-HPT patients and normals. Moreover, the pB% displayed a nearly Gaussian distribution pattern from 20% to over 90% in patients with either 1°-HPT or uremia. The specificity of this newly developed whole PTH(1–84) IRMA is the assurance, for the first time, of being able to measure only the biologically active whole PTH(1–84) without cross-reaction to the high concentrations of the aminoterminally truncated PTH fragments found in both normal subjects and patients. Because of the significant variations of pB% in patients, it is necessary to use the whole PTH assay to determine biologically active PTH levels clinically and, thus, to avoid overestimating the concentration of the true biologically active hormone. This new assay could provide a more meaningful standardization of future PTH measurements with improved accuracy in the clinical assessment of parathyroid function.