The experience at Hiroshima and Nagasaki can be correlated with clinical information from other sources (reactor accidents, exposures to radioactive fall-out, and experience with patients undergoing radiotherapy) to predict some of the hospitalization needs of a population in the event of a nuclear holocaust. Victims can be classified as to the form of acute radiation syndrome they manifest, whether cerebral, gastrointestinal, or hematopoietic. Deterioration is fastest, the prognosis worst, and hospitalization least helpful in patients with the cerebral form. The hematopoietic form, by contrast, has a latent period of about three weeks between recovery from the prodromal sickness and onset of the leukopenia and hemorrhage; the prognosis is best and hospitalization most likely to be effective. The gastrointestinal form is intermediate as to duration of latent interval, likelihood of recovery, and effectiveness of hospital care. The hematopoietic form of acute radiation sickness is likely to pose the major problem. The height of hospital occupation then should be reached during the fifth and sixth weeks after exposure, and medical authorities will probably have sufficient time to plan the necessary countermeasures.