The present invention concerns methods for diagnosis and treatment of metabolic bone diseases and disorders using a novel member of the tumor necrosis factor (TNF) family of cytokines. In particular the invention provides methods of using the Endokine alpha protein and/or homomultimeric and/or heteromultimeric polypeptide complexes containing Endokine alpha, in the diagnosis, prognosis and treatment of metabolic bone diseases and disorders. Also provided by the invention are methods of using the Endokine alpha protein and/or homomultimeric and/or heteromultimeric polypeptide complexes containing Endokine alpha, in the diagnosis, prognosis and treatment of diseases and/or disorders associated with aberrant osteoclast development and/or activity. The present invention also provides isolated polynucleotides encoding polypeptides of the invention, antibodies thereto, and agonists and antagonists thereof, for use in the diagnosis, prognosis and treatment of metabolic bone diseases and disorders.