Low PTH secretion is known to be associated with Adynamic Bone Disease (ABD). Positive balance calcium by CaCO3 or dialysate calcium (DCa) might play a role in the parathyroid gland suppression and a decrease in DCa to 2.5 mEq-l or lower has been proposed. The long-term effect of this procedure on bone mineral density (BMD) has not been established. The aim was to evaluate the effect of lowering dialysate calcium on bone mass in patients with relative hypoparathyroidism. We studied 20 patients with intact PTH below 120 pg/ml, using 3 mEq/l DCa and CaCO3 as sole phosphate binder. Sex: 10M/10F. Age: 57 +/- 13 yrs. Months on dialysis: 40 +/- 29. None of them had previous renal transplantation, parathyroidectomy nor aluminic toxicity. BMD of the lumbar spine was assessed by Quantitative Computed Tomography (QCT). They were randomized in two groups (GI and GII), with similar age, sex, and time on dialysis. There were no difference in BMD, levels of intact PTH, serum calcium, phosphate and AP (Alkaline Phosphatase) GI (n = 11; 5M/6F) was transferred to 2.5 mEq/l DCa and GII (n = 9; 5M/4F) continued using 3 mEq/l. BMD was measured one year later. Calcium, phosphate and AP were measured monthly and PTH every three months. After one year of hemodialysis with 2.5 mEq/l of calcium dialysate, BMD showed a significant reduction. BMD mg/cc Baseline (B): 146.09 +/- 54; Final (F): 125.42 +/- 54 (p < 0.01). Z-score B: 0.13 +/- 1.89; F: -0.68 +/- 1.89 (p < 0.05). GII did no show change. The mean change: GI: -15 +/- 13%, GII: 1.28 +/- 17% (p < 0.05); Z-Score GI: -0.81 +/- 0.92, GII: 0.27 +/- 0.67 (p < 0.01). A separate analysis of BMD in both sexes (GI) revealed a tendency for females to lose more bone mineral than males: F: = 17.12 +/- 7.1%. M: -12.23 +/- 18.6% (ns). GI: PTH and AP increased: PTH B: 38.75 +/- 41; F: 99 +/- 69 (p < 0.01); AP: B: 118.4 +/- 47; F: 152 +/- 38 (p < 0.01). GII: PTH B: 53.8 +/- 28; F: 79 +/- 5 (ns). AP: B: 125.1 +/- 36; F: 138 +/- 38 (ns). The rate of BMD loss inversely correlated with the increase of PTH (r = -0.61, p < 0.01). Serum calcium and phosphate did not change. In GI CaCO3 doses were: B: 332 +/- 261; F: 537 +/- 260 (as grams of element calcium, every three months, p < 0.01). By multiple lineal regression only delta PTH and DCa were predictors of greater BMD loss. In conclusion, the use of 2.5 mEq/l dialysate calcium resulted in: 1) Loss of trabecular vertebral bone mass. 2) Increase in PTH secretion and biochemical markers of bone formation. 3) A greater CaCO3 dose.