CONTEXT The positive association of elevated fibroblast growth factor-23 (FGF23) with PTH levels in the setting of secondary hyperparathyroidism is paradoxical to the purported effects of FGF23 to suppress PTH secretion. OBJECTIVE We used dynamic calcium-mediated suppression of PTH levels in hemodialysis (HD) patients to determine the relationship between FGF23 levels and parathyroid gland function. DESIGN HD patients with elevated PTH were washed out of vitamin D analogs and/or calcimimetics and then exposed them to a high-calcium dialysate bath designed to suppress PTH. SETTING The study was conducted at an outpatient HD unit of an academic medical center. PARTICIPANTS Eighteen maintenance HD patients with elevated PTH levels participated in the study. MAIN OUTCOME MEASURES Ionized calcium (iCa), PTH, and FGF23 levels were measured during HD. The slope of the relationship between iCa and PTH (a marker of parathyroid gland mass) and the iCa level required for a 50% reduction in PTH were determined, and the association of these with FGF23 levels was determined. RESULTS Increased baseline log FGF23 levels were associated with putative alterations in gland mass as estimated by significantly shallower slopes of the iCa/PTH suppression curves (P = 0.0004), but there was no association between FGF23 and calcium sensing as measured by ionized Ca associated with a 50% suppression of PTH (P = 0.38). FGF23 levels decreased significantly during HD, but this change was not correlated with decrements in either renal phosphate or PTH. CONCLUSIONS High FGF23 levels may be a marker for parathyroid gland hyperplasia in HD patients. Acute reductions in neither PTH nor renal phosphate during dialysis correlated with PTH suppression.