In the treatment of osteoporosis, M. Kahler and bone metastases from prostate and breast cancer bisphosphonates play a major role. Not all patients respond well to bisphosphonate treatment. Since a few years adverse effects of these drugs have been reported. A new drug, denosumab, a fully human monoclonal antibody to RANKL, has recently been developed. This case reports a 74-year-old male patient with a medical history of diabetes mellitus, angina pectoris, coronary bypasses, hypertension, and prostate cancer with multiple metastases to lymph nodes, bone and lungs. The prostate cancer was treated according to the protocol. But he was never treated with bisphosphonates. Instead he was included in a phase III randomized double blind multicenter trial, testing the efficacy of denosumab compared to zoledronic acid in the treatment of bone metastases of hormone resistant prostate cancer. Only 7 months after start of denosumab infectious symptoms developed, followed by infestation of the mandible. Despite surgical treatment fistula and exposed bone remained. This case illustrates that use of denosumab can lead to a type of osteonecrosis resembling bisphosphonate related osteonecrosis of the jaws.