This case describes a man with an unusual cause of an atypical subtrochanteric fracture, pycnodysostosis. This condition results from mutations involving the cathepsin K gene. New antiresorptive treatments for osteoporosis inhibit the cathepsin K enzyme in osteoclasts. Therefore, there should be vigilant monitoring for the development of long-term complications noted to occur in diseases of reduced osteoclast function, including pycnodysostosis, in patients receiving these novel antiresorptive agents. © 2011 American Society for Bone and Mineral Research.