Nineteen recently diagnosed patients with acute leukemias received prophylactic platelet transfusions when thrombocytopenic. Platelet concentrates were always transfused within eight hours of collection. The result of each transfusion was carefully monitored for the posttransfusion platelet increment. One patient died four days after admission to the present study. Seven patients died 1 to 14 months after the institution of antileukemic chemotherapy. Four patients received marrow transplants 1 to 2 months after starting chemotherapy and, therefore, were longer evaluated in this study. One patient was lost to follow-up after seven months of observation. The remaining six patients are alive and have been followed for 9 to 18 months. During the period of observation, none of the patients developed any evidence of refractoriness to platelet transfusion therapy. Although this series does not permit any firm conclusions to be made, it appears that the magnitude of the problem of alloimmunization to platelet transfusion therapy may be overemphasized.