Whole body bone scan and abdominal computed tomography (CT) of a 77-year-old man with intractable back pain revealed a destructive bone lesion in the lumbar spine. The F-18 FDG PET/CT scan, performed under the impression of an unknown primary cancer, showed diffusely intense FDG uptake in the enlarged prostate and mild FDG uptake in the lumbar spine. The patient had pyuria and mild fever during admission. Laboratory findings showed elevation of the serum prostate specific antigen (95.69 ng/ml). Blood culture yielded Klebsiella pneumoniae. The prostate biopsy showed acute inflammation with no malignancy and the lumbar spine biopsy showed chronic osteomyelitis.