The efficacy and safety of calcium carbonate as a phosphate binder was evaluated in 20 patients on chronic hemodialysis who had previously received aluminum hydroxide. During the control period the patients were on aluminum hydroxide and calcitriol therapy and had plasma phosphorus levels less than 6 mg/dL (4.95 +/- 0.8 mg/dL). Aluminum hydroxide was then discontinued and no phosphate binder was prescribed for 1 month. Every patient developed hyperphosphatemia so that calcium carbonate treatment was begun and calcitriol dose was adjusted in relation to plasma calcium changes. After 24 months of calcium carbonate therapy, plasma phosphorus was 4.85 +/- 0.7 mg/dL, using a daily dose of calcium carbonate of 2.57 +/- 1.3 g (range, 1 to 6 g). The daily dose per patient of calcitriol was not different from that prescribed during the control period, but in five patients calcitriol was permanently withdrawn for hypercalcemia. At the end of the study plasma calcium, magnesium, bicarbonate, alkaline phosphatase, and parathyroid hormone values were unchanged in comparison with the control period, whereas a significant reduction in plasma aluminum and plasma aluminum increase induced by deferoxamine infusion was observed. The frequency of hypercalcemic and hyperphosphatemic episodes during the last 12 months of calcium carbonate therapy (6.2% and 16.6%, respectively) was not different from that observed during the 12 months on aluminum hydroxide therapy preceding the control period (4.5% and 14.7%, respectively). It was concluded that calcium carbonate is effective in the control of hyperphosphatemia and secondary hyperparathyroidism in patients on chronic hemodialysis and that the incidence of hypercalcemia is low when the daily dosage is less than 6 g.