The intramedullary infiltration type, cell morphology and the pathological condition of bone lesion are closely related to diagnostic and clinical features in multiple myeloma. They are important factors to evaluate treatment effects and prognosis and to determine therapeutic strategy. In addition to various clinical parameters to determine prognosis, including M protein level, they should be minutely observed because the obtained findings provide useful basic data for monitoring multiple myeloma. In the future, much progress should be made towards identifying various factors which regulate dynamics of myeloma cell proliferation and bone remodeling.