The objective of this study was to elucidate the effect of oral calcium supplement upon the severity of secondary hyperparathyroidism and hyperphosphatemia in patients undergoing hemodialysis. Twenty-three accepted to participate and were randomly allocated to receive in a double-blind manner either 2 g elemental calcium per day (n = 12) or placebo (n = 11) for 6 months. Three patients dropped out leaving 10 patients in each group. In the calcium group serum ionized calcium increased significantly during the 1st month from 1.15 ± 0.02 to 1.29 ± 0.04 mmol/l (mean ± SE) and was then stabilized at an average of 1.26 ± 0.02 mmol/l for the remaining 5 months. Serum intact parathyroid hormone (PTH) decreased in average by 54% within the 1st month and remained at that level for the following months. Hyperphosphatemia remained stable throughout. On the contrary, in the placebo group, serum concentrations of ionized calcium and intact PTH remained unchanged, while serum phosphate increased significantly. The present study demonstrates that oral calcium reduces secondary hyperparathyroidism and prevents the progression of hyperphosphatemia in patients undergoing hemodialysis.