Fourteen patients with idiopathic thrombocytopenic purpura (ITP) have been followed for one year after plasma exchange therapy. Exchange was performed prior to splenectomy in eight of nine patients with acute ITP and following splenectomy in five patients with chronic ITP. None with chronic ITP showed a response in platelet count as a result of exchange therapy. Four of the nine with acute ITP had poor responses and required splenectomy because of persistent severe thrombocytopenia. Three of these have responded completely and are in remission. One patient with acute ITP had an equivocal response, with a most recent platelet count of 93,000/microliter. The remaining four patients with acute ITP had prompt and complete responses and now have platelet counts above 100,000/microliter without steroid treatment. Although the good responses were temporally associated with the use of plasma but not albumin, the data are not sufficient to conclude that a plasma factor must be infused to obtain a satisfactory result. The overall response after one year was about that expected for acute ITP patients treated with prednisone and early splenectomy. Exchange plasmapheresis may be of value in decreasing the number of patients who require splenectomy, but a randomized, prospective study is needed to adequately assess this possibility.