Atypical femoral fracture (AFF) has been widely issued since the American Society for Bone and Mineral Research (ASBMR) taskforce stated a possible association with long-term use of bisphosphonate in 2010. ASBMR taskforce recommended to include Paget's disease as one of variables to investigate the AFF in 2010. On the other hand, the ASBMR taskforce in 2014 excluded Paget's disease in the definition of AFF. Still, any rationale has been barely documented about this change. We described the rationale by reporting an 85-year-old man who had a subtrochanteric fracture with features of atypical femoral fracture which turned out to be Paget's disease of bone in the pathology.