We have discussed in an earlier paper the value of estimating the circulating levels of biologically active parathyroid hormone. We consider here the importance of an evaluation of circulating calcitonin by showing the frequency of raised calcitonin secretion in hypercalcaemia of different origins. These results lead one to attribute to calcitonin a role which goes beyond the regulation of phospho-calcium metabolism and which in fact is that of a particularly sensitive indicator of tumours.