A 68-year-old man was admitted for bleeding tendency and generalized lymphadenopathy. A diagnosis of idiopathic thrombocytopenic purpura (ITP) associated with mantle cell lymphoma was made. The anti-CD20 monoclonal antibody rituximab 375 mg/m2 was given intravenously once weekly for four consecutive weeks. The patient's platelet counts increased gradually from 0.6 x 10(4)/microliter to 5.8 x 10(4)/microliter. At eighteen weeks after discontinuation of rituximab medication, his platelet count increased again to 10. 3 x 10(4)/microliter and this value has been sustained up to the time of writing. This suggests that rituximab is useful in the treatment of ITP associated with malignant lymphoma.