Prostate cancer coexisting with asymptomatic Paget's disease (osteitis deformans) may be difficult to stage. The skeletal lesions of both prostate cancer and Paget's disease may closely resemble each other, as appearing on imaging. A case of clinically localized prostate cancer coexisting with incidental Paget's disease is herein reported. Prostate cancer and coexisting incidental Paget's disease both need careful evaluation and close follow-up of the skeleton by imaging, because there is a risk of the former involving the skeleton at low clinical stages and low PSA serum levels, as well as the latter developing osteosarcoma.