Autoimmune thrombocytopenic purpura (AITP) is generally a chronic disorder in affected adults. Twenty-five percent of these patients will become refractory to routine therapy (corticosteroids and splenectomy), as well as most other available agents. Intravenous pulse cyclophosphamide therapy was used to treat 20 patients with severe refractory AITP who had previously failed to achieve a sustained remission with a mean of 4.8 agents (range 2 to 8). Patients received 1 to 4 doses (mean 2.0) of 1.0 to 1.5 g/m2 intravenous cyclophosphamide per course. Of the 20 patients treated with pulse cyclophosphamide therapy, 13 patients (65%) achieved a complete response (CR), four (20%) a partial response (PR), and three patients (15%) failed to respond. Of the 13 complete responders, eight have remained in remission with stable platelet counts during followup intervals of 7 months to 7 years (median 2.5 years). Five patients developed recurrent AITP 4 months to 3 years following a CR. Of these, two patients responded to subsequent courses of pulse cyclophosphamide therapy with current remissions of 1 and 4 years. Of the four patients who obtained a PR, two remain in partial remission after 10 months and 4 years; one relapsed after 18 months and, after retreatment, is still in remission at 6 months. Of the patient characteristics examined, duration of disease was most strongly associated with response to pulse cyclophosphamide. Side-effects of treatment included neutropenia (three patients, one of whom developed staphylococcal sepsis), acute deep venous thrombosis (two patients), and psoas abscess (one patient). Intravenous pulse cyclophosphamide should be strongly considered in the treatment of patients with refractory AITP. There is a relatively low incidence of side-effects, and it can be administered easily on an out- patient basis.