Treatment modalities of patients with chronic immune thrombocytopenic purpura (ITP) include the administration of intravenous immunoglobulins (IVIG), corticosteroids, anti-D(Rh) immunoglobulin (anti-D), and splenectomy. Approximately 25-30% of patients with chronic ITP do not respond to established therapeutic regimens. We describe a 19-year-old patient with chronic ITP refractory to standard therapies treated with rituximab (anti-CD20 antibody). Initially, the therapy with rituximab appeared to be successful; however, the patient relapsed after a surveillance of 57 weeks documenting that the rituximab therapy has failed. Flow cytometric analyses during and after the administration of rituximab revealed new aspects of monitoring rituximab therapy.