A 75-year-old woman had an episode of sudden nasal and oral bleeding. After that, petechiae appeared on her entire body. She received a platelet transfusion, and was referred to our hospital. On admission, the platelet count was as low as 1.2 X 10(4)/microl, and the PAIgG level was slightly elevated. Bone marrow cellularity was low, with a normal count of megakaryocytes. Anti-glycoprotein IIb/IIIa antibody-secreting B cells in the peripheral blood and platelet-associated anti-glycoprotein IIb/ IIIa antibodies were significantly high, and the patient was diagnosed as having idiopathic thrombocytopenic purpura (ITP). She failed to respond to corticosteroids, splenectomy and other therapies, so we administered rituximab, anti-CD20 monoclonal antibody, 375 mg/m2 weekly for four weeks. After the second infusion of rituximab, the platelet count began to increase. The platelet count continued to rise until a peak count (15.0 x 10(4)/microl) observed after 2 weeks from the fourth infusion, and the response was maintained for 8 more weeks. The levels of anti-glycoprotein IIb/IIIa antibody-secreting B cells and platelet-associated anti-glycoprotein IIb/IIIa antibodies decreased after the administration of rituximab. Rituximab was effective in this case of refractory ITP.