Denosumab is a monoclonal antibody that can be administrated subcutaneously. Although it is not recommended to adjust the dosages for patients with impaired renal function, hypocalcemia has been reported in patients with renal impairment; therefore, it should be administered cautiously. We retrospectively investigated the serum concentrations of calcium after denosumab administration. The results indicated that after continuous administration to patients with a Ccr<40mL/min, serum calcium levels decreased. Grade 2 or above hypocalcemia was detected in 75% of the patients studied. From these results, it is recommended that serum concentrations of calcium be closely monitored in continuous administration of denosu- mab to patients with renal impairment.