We report a patient with severe hypercalcemia and acute kidney failure, in whom citrate anticoagulation was used not only for anticoagulation but also to correct ionized hypercalcemia (1.77 mmol/L). In this patient, after a complicated surgical procedure, septic shock led to acute kidney failure. We started continuous venovenous hemodialysis with citrate anticoagulation. By almost stopping the calcium substitution during the first hours, elevated systemic ionized calcium decreased into the normal range within 8 hours. Although calcium substitution was then increased, serum ionized calcium decreased to a nadir of 0.86 mmol/L and then stabilized within the normal range within the next 24 hours. To correct the imbalance in systemic ionized calcium concentration, the calcium substitution was varied over a wide range of 0.1-3.0 mmol/L of generated effluent. The time delay between adjustment in calcium infusion rate and the first detectable change in ionized calcium level was below 4 hours. However, the full response to a change of the calcium substitution was found after 8-12 hours.