To develop guidelines for the diagnosis and management of idiopathic thrombocytopenic purpura (ITP) and to document the extent to which those guidelines are based on either scientific evidence or opinion, the AMerican Society of Hematology established a panel composed of 13 hematologists with expertise in ITP, a clinical epidemiologist, and a practice guidelines methodologist. A comprehensive review was done of all published English-language studies that met explicit inclusion criteria and that evaluated the natural history of ITP or the effectiveness of testing and treatment options for ITP. The quality of each study was graded by two reviewers using formal methodologic rules. In subject areas for which data was inadequate, recommendations were based on opinion and were derived by using a formal screening procedure. Confidential questionnaires were used to survey the hematologists on the panel about the appropriateness of testing and treatment options in hundreds of clinical scenarios. Practice recommendations were derived from the mean appropriateness scores for each indication. Voting was kept confidential to give each panel member an equal voice and to limit biases introduced by group dynamics. The recommendations were peer reviewed by eight outside experts. This report focuses on data and on recommendations for adults with ITP. Little high-quality scientific evidence with which to assess the efficacy of diagnostic tests and treatments for ITP is available. The opinion of the panel was that most diagnostic tests are unnecessary in the routine work-ups of patients suspected of having ITP and that ITP accompanied by severe bleeding requires treatment with glucocorticoids, intravenous immunoglobin, and other measures. However, treatment and hospitalization is often unnecessary when patients have only mild or moderate thrombocytopenia or minimal bleeding. Special therapeutic measures are sometimes indicated in pregnant women with ITP.