The recent comprehensive reports on the effects of ionizing radiation published almost simultaneously by the National Academy of Sciences in the United States and by the Medical Research Council in England1have inevitably led to a reassessment of the hazards of present-day diagnostic and therapeutic x-ray procedures. There has already been some comment inThe Journalon the subject, although the matter of leukemogenesis was not particularly stressed.2It can be stated that, of the various etiological factors proposed for leukemia in human beings, the only one that appears to be rather conclusively established is that of ionizing radiation.3The evidence for this includes (a) the incidence of leukemia in radiologists, which is 8 to 10 times as great as that in physicians in other fields4; (b) the incidence of leukemia in those surviving the atomic blasts near the hypocenters of Hiroshima and Nagasaki, which is