Objective: To determine the most effective, evidence-based approach to the use of platelet transfusions in patients with cancer. Outcomes: Outcomes of interest included prevention of morbidity and mortality from hemorrhage, effects on survival, quality of life, toxicity reduction, and cost-effectiveness. Evidence: A complete MedLine search was performed of the past 20 years of the medical literature. Keywords included platelet transfusion, alloimmunization, hemorrhage, threshold and thrombocytopenia. The search was broadened by articles from the bibliographies of selected articles. Values: Levels of evidence and guideline grades were rated by a standard process. More weight was given to studies that tested a hypothesis directly related to one of the primary outcomes in a randomized design. Benefits/Harms/Cost: The possible consequences of different approaches to the use of platelet transfusion were considered in evaluating a preference for one or another technique producing similar outcomes. Cost alone was not a determining factor. Recommendations: Appendix A summarizes the recommendations concerning the choice of particular platelet preparations, the use of prophylactic platelet transfusions, indications for transfusion in selected clinical situations, and the diagnosis, prevention, and management of refractoriness to platelet transfusion. Validation: Five outside reviewers, the ASCO Health Services Research Committee, and the ASCO Board reviewed this document. Sponsor: American Society of Clinical Oncology.