Recent in vitro and in vivo studies have shown that calcium acetate (CaAC) is a more effective phosphorus binder than, among other calcium salts, calcium carbonate (CaCO3). More efficient binding allows serum phosphorus to be controlled with a lower dose; moreover, less calcium seems to be absorbed when CaAC is used. These properties could reduce the incidence of hypercalcemia; however, in clinical practice few reports have compared these two calcium salts, and results disagree. We evaluated in a 24-week prospective crossover study the clinical efficiency of CaCO3 and CaAC in 10 selected chronic hemodialysis patients. Only 7 patients completed the study period. The patients were randomly assigned to start treatment with one of the two calcium salts; after 12 weeks they shifted to the other treatment. Serum analytical tests included weekly control of calcium, phosphorus, and alkaline phosphatase. PTH values (intact molecule) were obtained initially and at the end of every study period. The same good control of the phosphorus level (4.79 ± 0.6 vs. 4.94 ± 0.8 mg/dl) was obtained with CaAC (mean doses 4.1 ± 0.3 g/day) as with CaCO3 (mean doses 4.01 ± 0.8 g/day). The mean serum calcium levels were similar (10.36 ± 0.5 vs. 10.20 ± 0.5 mg/dl). The dose of elemental calcium administered was significantly less with CaAC (957 ± 83 mg/day) than with CaCO3 11 mg/dl) was similar during the two treatment periods (13% with CaAC vs. 14% with CaCO3 65 was comparable (9.5 vs. 11.9%). In conclusion: 40% less calcium needs to be supplied when CaAC is used; however, the control of hyperphosphatemia and the frequency of hypercalcemia was the same as with CaCO3. There is no clear advantage of CaAC over CaCO3 in a medium-length (24 weeks) comparativ