Platelets were removed from 25 patients with leukemia during remission and were frozen for subsequent transfusion. With 5 per cent dimethyl sulfoxide as a cryoprotective agent, we froze 3 to 5 units of pooled platelet concentrate by simply placing the platelets in the vapor phase of a liquid nitrogen freezer. Ninety-one transfusions of platelets stored for 13 to 400 days were administered. The mean freeze-thaw loss was 13 percent, and the corrected one-hour increment in platelet count was 13,700 per microliter, corresponding to a recovery of 53 per cent of the predicted value. In many patients most or all of the transfusion requirements were met with frozen platelets. Our results indicate that frozen platelets can circulate and function hemostatically. Autologous frozen platelets are of particular value in the management of alloimmunized patients and have become an integral part of our transfusion support program.