In a recent series of 110 cases of primary hyperparathyroidism, estimations of plasma immuno-parathormone (PTH) were carried out in fifty two patients. This estimation proved reliable, making possible the confirmation of the diagnosis. In the absence of renal insufficiency, there was a highly positive relationship between PTH levels and plasma calcium. The relationship between PTH and the weight of the parathyroid tumour was less significant. For technical reasons, this long and difficult estimation cannot be used on a routine basis. However, it is most useful in cases in which other laboratory findings are not typical.