Platelet concentrates stored at 2 degrees to 4 degrees C or at 20 degrees to 24 degrees C for 24 or 72 hr were transfused to 22 thrombocytopenic patients selected to minimize host factors which affect the survival and function of transfused platelets. Platelets stored at either temperature range for only 24 hr were equally effective in elevating platelet levels and in shortening bleeding times, 1, 4 and 24 hr after transfusion. After 72 hr of storage, however, refrigerated platelets were significantly less effective than platelets kept at room temperature by these two criteria. We suggest that routine storage of platelets at 2 degrees to 4 degrees C can no longer be justified except possibly when platelets are to be shipped over long distance by common carrier and transfused within 24 hr of preparation.