Because of its B-cell depleting effect, rituximab has entered clinical trials in several autoimmune conditions. This study assesses the efficacy and safety of rituximab in 57 adults with chronic immune thrombocytopenic purpura (ITP). All patients had platelet counts < 30 x 10(9)/l, all had received two or more previous ITP treatments and 31 had undergone splenectomy. Patients received rituximab 375 mg/m(2) weekly for 4 weeks. Thirty-one patients (54%) responded, achieving a platelet count >50 x 10(9)/l: 18 achieved a complete response (CR: platelet count >150 x 10(9)/l) and 13 a partial response (PR: platelet count 50-150 x 10(9)/l). Twenty-nine responses occurred within 8 weeks of the first infusion. Sixteen of 18 CR patients (28% overall), including eight who had failed splenectomy, continued in CR after a median of 72.5 weeks; 15 of 16 are >1 year from the first infusion. Only two of 13 maintained a PR. Thirty-three patients experienced grade 1-2 adverse events and one a grade 3 event, but they all completed treatment. Circulating B cells fell to < 0.03 x 10(9)/l. No changes in immunoglobulin levels or infectious complications were seen. In summary, rituximab was well tolerated with no immediate complications and induced a lasting, substantial response in 32% of adults with chronic ITP.