Background: Renal osteodystrophy is one of the major complications in patients with chronic renal failure. Large C-PTH fragments are secreted from the parathyroid glands and exert antagonistic actions against PTH( 1 - 84). The PTH-(1 - 84)/ large C-PTH fragments ratio reflects both biosynthesis and processing of PTH; however the alteration of the ratio under vitamin D therapy has not been investigated. Methods: Seventeen hemodialysis patients with intact PTH levels of > 300 pg/ml were enrolled. Calcitriol or maxacalcitol were administered intravenously for 78 weeks. Intact PTH, PTH-( 1 - 84), and the PTH-( 1 - 84)/ large C-PTH fragments ratio were measured at 0, 13, 26, 52 and 78 weeks. Results: Intact PTH and PTH-(1 - 84) levels, which were 492.0 +/- 115.7 and 303.4 +/- 105.4 pg/ml, respectively, at baseline, significantly decreased at the end of the study to 268.9 +/- 121.9 ( p < 0.0001) and 190.7 +/- 106.9 pg/ml ( p = 0.0008), respectively. In contrast, large C-PTH fragments, which were 152.7 +/- 53.5 pg/ml at baseline, did not significantly change at 78 weeks (144.5 +/- 72.2 pg/ml, p = 0.7612). Consequently, the PTH-(1 - 84)/ large C-PTH fragments ratio was significantly reduced from 2.25 +/- 1.31 to 1.47 +/- 0.89 (p = 0.0004). Conclusion: The PTH-(1-84)/ large CPTH fragments ratio reflects the change of PTH biosynthesis, processing and secretion from the parathyroid glands, and it may be a beneficial marker to evaluate the overall biological PTH action and predict bone turnover status in hemodialysis patients under intravenous vitamin D therapy. Copyright (C) 2004 S. Karger AG, Basel.