Paget's disease of the bone is the second-most-common metabolic bone disease. However, it is rarely found in the Far East. Vertebral Paget's disease usually occurs at multiple vertebral levels, with less than 20% of vertebral Paget's disease being monostotic. Here, we present a rare case of monostotic vertebral Paget's disease, which we initially misdiagnosed as a spinal metastasis. A 34-year-old man was admitted with a one-month history of lower back pain. Initially, computed tomography of the lumbar spine showed an osteolytic change of the L4 and mild expansion of the L4 vertebral body. Subsequently, magnetic resonance imaging showed a highly homogenously enhanced L4 vertebral body. We performed positron-emission tomography, bone scan, and tumor marker evaluation. However, we could not detect any likely primary origin of the spinal metastasis. Therefore, we carried out a needle bone biopsy under fluoroscopic guidance, and the bone specimen revealed Paget's disease of the bone.
