Rituximab 375 mg/m(2) weekly for four weeks has significant activity in patients with immune thrombocytopenia. We evaluated the activity of lower dose rituximab (100 mg iv weekly for 4 weeks) in 28 adults with idiopathic thrombocytopenic purpura. Overall (platelet count > 50 x 10(9)/L) and complete responses (platelet count > 100 x 10(9)/L) were achieved in 21/28 (75%) and 12/28 (43%) patients respectively. The median time to response and time to complete response were 31 and 44 days respectively. After a median follow-up of 11 months (range 3-18), 7/21 (33%) patients relapsed and 3 needed further treatments. In patients with idiopathic thrombocytopenic purpura, lower dose rituximab seems to show similar activity to standard dose.