Cyclophosphamide, an immunosuppressive agent, was administered as an additional mode of therapy to 30 patients with idiopathic thrombocytopenic purpura (ITP) refractory to conventional management. Of 22 previously tested by splenectomy an excellent response was achieved in 12, who remained in complete hematologic remission for 14–96 months after therapy was discontinued; a fair response in 3, with definite increase in platelets, but not to normal levels; and a poor response in 7 who failed to improve. Of 8 nonsplenectomized patients who failed to respond to steroids or maintain a response after steroids were discontinued, 4 were considered excellent, 1 required continued therapy to remain in remission (good response), 2 were fair, and 1 was poor. Remission was observed in 2–10 weeks in both groups and appeared to be related to duration of disease; presence of disease for less than 1 year was associated with a much better response to treatment (11 of 15) when compared with disorders lasting over 2 years (6 of 15). Cyclophosphamide therapy offers additional means of treating patients with ITP who fail to respond to conventional therapy and may serve as an alternative to splenectomy when surgery is contraindicated.