A 60-year-old man went to his local orthopedic surgeon for right lower limb pain. The symptoms started 3 months previous. There was no history of trauma and past medical and family history were unremarkable. Plain radiographs of the lumbar spine showed degenerative spondylosis of the lower lumbar spine. Computed tomography (CT) and magnetic resonance imaging (MRI) of the lumbar spine showed mild right posterolateral L4-L5 and L5-S1 disc protrusion. With the presumptive diagnosis of lumbar disc herniation, nonsteroidal antiinflammatory drug therapy and physiotherapy were recommended. A month later, clinical examination showed painful restriction of right hip range of motion. Laboratory tests, including complete blood count, serum chemistries, serum immunoelectrophoresis, and erythrocyte sedimentation rate, were all normal. Plain radiographs (Fig 1), bone scintigram (Fig 2), and MRI scans (Fig 3) were obtained. Based on the history, physical examination, laboratory tests, and imaging studies, what is the differential diagnosis? From the *First Orthopaedic Department, Athens University Medical School, Athens, Greece; the †Division of Medical Oncology, Mayo Clinic, Rochester, MN; and the ‡Department of Pathology, Hygeia Athens Medical Center, Athens, Greece. Each author certifies that he or she has no commercial associations (e.g., consultancies, stock ownership, equity interest, patent/licensing arrangements, etc.) that might pose a conflict of interest in connection with the submitted article. Each author certifies that his or her institution has approved the reporting of this case and that all investigations were conducted in conformity with ethical principles of research. Correspondence to: Panayiotis J. Papagelopoulos, MD, DSc, First Department of Orthopaedics, Athens University Medical School, 4 Christovassili Street, Neo Psychikon, 15451 Athens, Greece. Phone/Fax: 011-30-210-6721355. E-mail: pjp@hol.gr. DOI: 10.1097/01.blo.0000194671.72011.2b CLINICAL ORTHOPAEDICS AND RELATED RESEARCH Number 443, pp. 342–349 © 2006 Lippincott Williams & Wilkins