Circulating parathyroid hormone (PTH) immunoheterogeneity is altered in primary hyperparathyroidism (pHPT). It is not known, however, whether the relative secretion of various PTH fragments differs between the adenomatous and the non-adenomatous glands in pHPT. We therefore examined the immunoheterogeneity of PTH in patients operated upon because of parathyroid adenoma pre- and 4 days postoperatively during an EDTA-infusion test. Following surgery, baseline levels of amino-terminal PTH (N-PTH) were reduced by a smaller degree than the levels of intact PTH (i-PTH) (P < 0.05) resulting in a higher N/i ratio postoperatively (P < 0.05). Furthermore, the increase in i-PTH and C-PTH fragments during the EDTA infusion test was lower postoperatively than preoperatively (P < 0.05), whereas the increase in N-PTH did not differ. The results therefore suggest that compared with the parathyroid adenoma, the non-adenomatous glands secrete relatively more N-terminal PTH.