In patients with either Paget's disease or hypercalcaemia associated with malignancy (HCM) we have assessed the parathyroid response to pamidronate therapy, both by immunoassay of serum intact parathyroid hormone PTH (1-84) and by measurement of indirect parameters of PTH bioactivity, tubular maximum reabsorption of phosphate (TmPO4/GFR) and nephrogenous cyclic AMP (NcAMP). In 12 patients with Paget's disease, therapy with pamidronate produced a small but significant decrease in adjusted serum calcium within the reference interval which was accompanied by a progressive increase in PTH (1-84) secretion and a corresponding fall in TmPO4/GFR and increase in NcAMP. In 12 patients with HCM pretreatment, PTH (1-84) concentrations were suppressed, whilst mean TmPO4/GFR was reduced and NcAMP was increased, compatible in most patients, with parathyroid hormone-related peptide (PTHrP) driven hypercalcaemia. Therapy with pamidronate produced the expected fall in serum calcium but caused an increase in PTH (1-84) secretion in the presence of absolute hypercalcaemia. The initial subnormal TmPO4/GFR decreased further to a nadir on day 5, and there was a corresponding further increase in NcAMP. By day 7, however, when PTH (1-84) concentrations were maximal, there was a significant paradoxical rise in TmPO4/GFR and a corresponding decrease in NcAMP. These data are consistent with a variable trigger point for PTH (1-84) secretion, one consequence of which is a reduction in the risk of hypocalcaemia following pamidronate. The results have major clinical implications for the interpretation of PTH (1-84) measurements in patients who are being treated or about to be treated for bone disease or for hypercalcaemia of malignancy (HCM).(ABSTRACT TRUNCATED AT 250 WORDS)