BACKGROUND: Prophylactic platelet (PLT) transfusion practices have become more conservative as studies support a threshold for transfusions at 10 × 109 per L. This change in practice may reduce our use of PLT transfusions. STUDY DESIGN AND METHODS: Data were prospectively collected to assess the impact at one academic hospital when the transition from a 20 × 109 to a 10 × 109 per L threshold prophylactic transfusion was made. RESULTS: A total of 503 patients received 7401 PLT transfusions. Seventy-four percent of the transfusions were prophylactic. During the first phase of the study, only 53 percent of transfusions were given at a pretransfusion PLT count of less than 20 × 109 per L and 20 percent less than 10 × 109 per L. In the second phase of the study when the transfusion trigger was 10 × 109 per L, 28 percent of transfusions were given at this level. CONCLUSION: Many prophylactic PLT transfusions were given at PLT counts higher than the recommended trigger. Although the new transfusion guidelines altered transfusion practice, only a minor change in overall PLT usage was observed. Other changes in transfusion practices, such as dose per transfusion or sampling interval, will be required before significant reduction in the costs and hazards of prophylactic PLT transfusions can be realized.