We compared the effectiveness of calcium acetate as a phosphate binder with that of calcium carbonate by substituting one for the other in patients undergoing continuous ambulatory peritoneal dialysis. Twenty patients who had been receiving calcium carbonate as a phosphate binder were instead given calcium acetate, initially with two thirds of the previous dose of elemental calcium. The calcium acetate dose was adjusted to achieve adequate calcium-phosphate balance; 65.6% of the previous dose of elemental calcium in calcium carbonate was required. Eighteen of the 20 patients completed the 3-month study. There were no significant differences in the pre-study and study levels of serum phosphate (1.81±0.04 [SEM] versus 1.89±0.06 mmol/L), corrected serum calcium (2.54±0.04 versus 2.57±0.03 mmol/L), calcium phosphate product (4.60±0.15 versus 4.87±0.18), serum alkaline phosphatase (64.75±4.17 versus 69.94±3.77 U/L), and serum parathyroid hormone (122±31 versus 124±27 ng/L). Three patients developed a total of five episodes of hypercalcaemia (corrected calcium level ≥2.85 mmol/L) and four other patients developed gastrointestinal upset. Calcium acetate can thus achieve similar phosphate control to calcium carbonate, using 65.6% of the dose of elemental calcium in calcium carbonate; how- ever, its clinical superiority was not demonstrated in this study.