The effect of intravenous 1α-hydroxyvitamin D3 [1α(OH)D3] on circulating levels of intact parathyroid hormone (PTH 1-84) and COOH-terminal immunoreactive PTH (PTH 53-84) was examined in 13 patients on chronic hemodialysis. Thirteen patients were treated for 300 days (10 months), 9 patients for 520 days (14 months) and 6 patients for 720 days (2 years) with increasing doses of 1α(OH)D3 intravenously under careful control of plasma Ca2+. Blood samples were obtained 1 week before start of treatment and then at every 2nd week. None of the patients had previously been treated with oral vitamin D metabolites. Intact PTH levels were maximally suppressed after 27-33 weeks of treatment by approximately 73 %. At the end of the study periods, PTH 1-84 was still suppressed by 78 ± 4.3% after 300 days, 78 ± 8.8% after 520 days and 85 ± 6.5% after 720 days. Plasma Ca2+ was kept within normal levels, but showed an initial increase from 1.14 ± 0.03 to 1.27 ± 0.15 mmol/l, and an adjustment of the doses of lα(OH)D3 was necessary. The present investigation demonstrated (1) that intravenous administration of the 1-hydroxylated vitamin D metabolite lα(OH)D3 induced a significant decrease in circulating levels of biologically active intact PTH, and (2) that it was possible to maintain the marked suppression of PTH secretion by intravenous treatment of 1α(OH)D3 for up to 2 years. Hypercalcemia could be avoided by careful monitoring of plasma Ca2+ and adjustment of the doses of 1α(OH)D3.