The use of platelet transfusions to prevent bleeding in patients with thrombocytopenia due to chemotherapy or other causes of marrow suppression is widespread, but the optimal number (dose) of platelets is unsettled. In this randomized trial, three doses of platelets were studied: the usual dose, half the usual dose, and twice the usual dose. No major differences in bleeding complications were found among the three groups, but more transfusions were given in the lowest-dose group to prevent bleeding. In this randomized trial, three doses of platelets were studied: the usual dose, half the usual dose, and twice the usual dose. No major differences in bleeding complications were found among the three groups, but more transfusions were given in the lowest-dose group to prevent bleeding. The optimal number of platelets in a prophylactic platelet transfusion is controversial. 1 , 2 A standard dose for adults is considered to be approximately 3×10 11 to 6×10 11 platelets. 3 Higher doses than these could potentially result in superior hemostasis, 4 but a lower dose might be equally effective while conserving the platelet supply. Two randomized trials with limited enrollment — one of 111 patients 5 and the other of 119 patients 6 — have compared a low dose of platelets to the standard dose. In both trials, the two doses prevented bleeding to a similar degree. We conducted a randomized trial of prophylactic platelet transfusions . . .