Systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) are representative autoimmune diseases thought to involve disturbances in T and B cell functions. Immune complexes consisting of antigens and autoantibodies secreted from activated B cells cause severe inflammation in various organs. Since often patients with RA and SLE are refractory to these conventional treatments such as immunosuppressants and corticosteroids, innovative approaches need to be developed. CD20 is a surface molecule specific for B cells and rituximab is a chimeric antibody specific for human CD20 and is known to deplete B cells. Recently, the potential efficacy of B-cell depletion therapy with rituximab has been reported in several autoimmune diseases. Rituximab is now approved for use in combination with methotrexate in refractory RA patients in the United States and EU. We reported that SLE patients with organ-threatening disorders that are resistant to intensive conventional therapies, were treated by once a week administration of anti-CD20 antibody rituximab, and that sufficient evidence concerning the excellent tolerability and high efficacy of rituximab therapy was obtained in both a pilot study and a nation-wide phase I/II clinical examination Moreover, a rapid and marked reduction in the expression of the co-stimulatory molecules CD40 and CD80 on B-cells was found in SLE patients, implying that reduction of both the quantity and the quality of B-cells by rituximab could improve the disease course in refractory SLE. Therefore, targeting B-cells may have potential interests by bringing about a breakthrough in the treatment of RA, SLE and other autoimmune diseases.