In a pilot study four patients with systemic lupus erythematosus (SLE) and autoimmune thrombocytopenia (ITP) were treated with rituximab, a B cell depicting chimeric human/mouse anti-CD20 antibody. Treatment could be performed without serious side effects and resulted in a depletion of B cells from the peripheral blood for at least 4 months. Examination of one patient three months after treatment revealed a complete depletion of B cells in the bone marrow and in the spleen as well. The time point when peripheral B cells returned into the normal range varied between 8 months and over one year and could be observed also in the spleen. The follow up over more than 12 months revealed no significant treatment-associated side effects. Total immunoglobulin and specific antibody levels did not change except for one SLE-patient receiving additional immunosuppressive treatment including cyclophosphamide because of progressive disease. Clinical effectiveness cannot be judged by the small number of patients. However, one SLE patient with refractory severe thrombocytopenia had a very Favourable response with stable platelet numbers over 100.000/microl now for more than 6 months and disappearance of anti-DNA antibodies. The treatment failure in another SLE patient could be due to the persistence of CD20-negative plasmablasts in peripheral blood which are not targeted by anti-CD20 treatment. Further studies are needed to assess the clinical benefit of B cell depletion in the treatment of autoimmune diseases.