In the past, patients with chronic thrombocytopenia died of hemorrhage with distressingly predictable frequency. The increased use of platelet transfusions during the past 15 years has prevented most such deaths. Furthermore, this therapy has made it possible to treat patients who have otherwise fatal disorders with drugs that temporarily suppress platelet production. With this great benefit, however, have come complex problems. Transfused platelets can transmit fatal diseases and can elicit an immune response in recipients, so that further transfusions are no longer effective. Although platelet therapy has contributed greatly to the treatment of patients with many diseases (eg, acute leukemia and aplastic anemia), serious questions have emerged regarding its use in patients undergoing cardiac surgery and in other circumstances. The prophylactic administration of platelets is also controversial, and there is uncertainty as to the platelet levels that predispose thrombocytopenic patients to hemorrhage and as to the effectiveness of modalities other than platelets in the prevention of bleeding. The relative merits of the various methods for obtaining and storing platelets remain unclear. The advantages and disadvantages of the use of platelets obtained from multiple and single donors require evaluation. To resolve these issues, the National Heart, Lung, and Blood Institute, the Center for Drugs and Biologics of the Food and Drug Administration, and the Office of Medical Applications of Research convened a Consensus Development Conference on Platelet Transfusion Therapy Oct 6 to 8, 1986. At a 1 1/2-day series of presentations by experts in the field, a consensus panel drawn from the medical professions, blood banking organizations, and the general public considered the evidence. The panel agreed on answers to the following key questions: What are the appropriate indications for platelet transfusion? What products are available, and what are their relative merits? What are the risks associated with platelet transfusion? What are the most important directions for future research?