Parathyroid hormone (PTH) concentrations were compared in serum and EDTA plasma from 36 patients attending a renal stone clinic. Serum PTH concentrations ranged from 0.9 to 10.9 pmol/L, with a mean of 4.6pmol/L. When serum and EDTA plasma results were compared, in samples frozen within 30 min of collection, EDTA plasma results were found to be significantly higher than those in serum (P < 0.0001; Wilcoxon test), with an average increase of 19.5% over the serum result. Results from EDTA-preserved blood left to stand at room temperature for 48 h were on average 14.8% lower than results from the corresponding EDTA plasma samples frozen within 30 min, with highly significant difference (P < 0.0001). Freshly frozen serum and 48h EDTA plasma PTH results were not significantly different. Parathyroid hormone in EDTA-preserved blood is not completely stable, and this could lead to misclassification of results for samples which are not frozen quickly.