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. 80 g of intravenous calcium and 370 g of oral calcium were administered in vain and the maximum level of ionized calcium achieved was 0.71 mmol/ during the first 16 days of hospital stay. Due to worsening renal failure due to tumor spread, our patient required dialysis to achieve normal calcium levels. Checking calcium and vitamin D levels before administration of Denosumab is very important to prevent this adverse outcome. By presenting this case we highlight the importance of checking vitamin D levels before administering the drug and also make other doctors aware of a potential serious and life-threatening electrolyte imbalance. Denosumab is a fully human monoclonal antibody, administered subcutaneously, that inhibits osteoclast-mediated bone resorption activity in bone metastases from solid tumors and multiple myeloma. See Figure A below for details. A 45-year-old gentleman 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 previously been treated with androgen deprivation therapy, along with 3 doses of zoledronic acid for bone metastases. The bone pain was not controlled with the above regimen and therefore he was switched to Denosumab. Vital values ​​on admission were within normal limits, with the exception of blood pressure of 160/90 mmHg. Pertinent findings on physical examination were the presence of bilateral lower extremity edema and negative Chvostek and Trousseaus signs. Review of symptoms was positive for hematuria and leg. 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›, cited 19 April 20135. Paller CJ, Carducci M, Philips G. Management of bone metastases in Refractory prostate cancer: role of denosumab. Clinic. 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 denosumab-treated patients with renal failure? Expert opinion. Biol. There., 13, 149–151 (2013).