Multiple myeloma (MM) is a lymphoproliferative disorder that is characterized by a proliferation of clonal B cells in various stages of maturation that then infiltrate the bone marrow. MM has been reported to accompany various T cell abnormalities including quantitative and functional defects of CD4+ and CD8+ T cells. Recently, immunotherapy such as dendritic cell therapy, vaccination therapy, and anti-tumor antibody therapy, has been attempted in patients with MM. To develop more effective immunotherapy for patients with MM, further studies are required to identify the immunological abnormalities, especially in T cells, associated with MM. The T helper 1 (Th1) and T helper 2 (Th2) cells are characterized by distinct cytokine production patterns. The Th1 cells produce interferon gamma and interleukin-2 (IL-2), and are involved in cell-mediated immunity. The Th2 cells produce IL-4 and promote humoral immunity by stimulating antibody production, particularly IgE responses. Furthermore, Th1 and Th2 cells have been found to cross-regulate each other's development. The Th1/Th2 combination has an important role in immune response to many disorders including infection, autoimmune diseases, and malignancies. In this review, we report a Th1/Th2 imbalance in cases of MM, and discuss the relationship between T cell abnormalities and the pathology of MM.