Background: The current study was undertaken to clarify the dynamic response of parathyroid hormone (PTH) during hemodialysis and to determine whether or not such dynamic change of PTH affects bone turnover. Methods: Serum ionized calcium (iCa) and intact PTH (iPTH) were measured in 66 dialysis patients before (basal) and after each hemodialysis. The changes of iCa (ΔiCa) and iPTH (ΔiPTH) were defined as [postdialysis iCa – basal iCa] m M, and [(postdialysis iPTH – basal iPTH)/basal iPTH] × 100%, respectively. We also investigated the data of the patients divided into four groups based on their basal iPTH levels; group 1 (iPTH <60 pg/ml, n = 17), 2 (≧60 to <150, n = 20), 3 (≧150 to <300, n = 15), and 4 (≧300, n = 14). Results: While iCa and iPTH changed variably during each hemodialysis procedure, there was a highly significant inverse correlation between ΔiCa and ΔiPTH (r = –0.761, p < 0.0001). Regression coefficients between ΔiPTH and ΔiCa were –519.1, –311.2, –268.1, and –194.6%/m M in groups 1–4, respectively. The difference in the regression coefficient was statistically significant between group 1 and 2 (F = 3.69, p < 0.05, ANCOVA), group 1 and 3 (F = 5.599, p < 0.05), and group 1 and 4 (F = 10.853, p < 0.005). This suggested that patients with higher basal iPTH responded poorly to modulate serum PTH levels by sensing the change of iCa. However, after an intensified oral calcitriol therapy to reduce iPTH, the PTH response in group 4 patients was restored to levels comparable with those observed in patients having lower basal iPTH. We also demonstrate that the ΔiPTH of patients of group 1 but not of other groups was significantly correlated with serum markers for bone metabolism; osteocalcin (r = 0.535, p < 0.05) and collagen type I C-terminal telopeptide (r = –0.575, p < 0.05). Conclusions: Our findings suggest that secretion of PTH is dynamically regulated by ΔiCa during hemodialysis and such oscillated PTH secretion may affect bone metabolism in a subset of dialysis patients.