Hypocalcemia is a rare and dangerous side effect of the drug denosumab. We present the case of a patient with metastatic prostate cancer who developed severe hypocalcemia after drug administration. A total of 80 g of intravenous calcium and 370 g of oral calcium were administered to no avail and the maximum ionized calcium level achieved was 0.71 mmol/during the first 16 days of hospitalization. Due to worsening kidney failure due to tumor spread, our patient required dialysis to achieve normal calcium levels. Checking calcium and vitamin D levels before administering denosumab is essential to prevent these adverse effects. By presenting this case, we highlight the importance of checking vitamin D levels prior to drug administration and also raising awareness among other clinicians of a potentially serious and life-threatening electrolyte imbalance. Denosumab is a fully human, subcutaneously administered monoclonal antibody that inhibits osteoclast mediation. Bone resorption in bone metastases of solid tumors and multiple myeloma. See Figure A below for details. A 45-year-old man with a 3-year history of metastatic prostate cancer (bone, liver, and lymph nodes) and hypertension presented to the hospital with worsening leg swelling and hematuria. He had already been treated with androgen deprivation, as well as 3 doses of zoledronic acid for bone metastases. Bone pain was not controlled with the above regimen and so it was replaced with Denosumab. Vital signs on admission were within normal limits, except for a blood pressure of 160/90 mmHg. Relevant physical examination findings were the presence of bilateral lower extremity edema and negative Chvostek and Trousseaus signs. Symptom review was positive for hematuria and leg...... middle of article ......ence and risk of denosumab-related hypocalcemia in cancer patients: a review systematic and pooled analysis of randomized controlled studies. Qi WX, Lin F, He AN, Tang LN, Shen Z, Yao Y.3. Severe hypocalcemia after denosumab injection in a hemodialysis patient Brendan B. McCormick, MD, Janet Davis, MD, and Kevin D. Burns, MD4. Pharmaceuticals and Medical Devices Agency. : ‹http://www.info.pmda.go.jp/kinkyu_anzen/file/kinkyu20120911_1.pdf›, accessed April 19, 20135. Paller CJ, Carducci M, Philips G. Management of bone metastases in prostate cancer refractory – role of denosumab. Clin. Interv. Aging, 7, 363-372 (2012).6. Buonerba C, Caraglia M, Malgieri S, Perri F, Bosso D, Federico P, Ferro M, Rizzo M, Palmieri G, Di Lorenzo G. Calcitriol: a better option than vitamin D in patients with renal failure treated with denosumab? Expert opinion. Biol. There., 13, 149–151 (2013).