The medical burden at Nagasaki i.e. the number of persons who required medical care as a result of receiving skin burns caused by thermal radiation, was determined as a function of the distance from ground zero and radiant exposure. The casualties were computed on the basis of available burn casualty and survival data and a re-estimate of burst conditions. Of an initial population of approximately 174,400 persons, 8355 or 4.5% of the total population survived 20 days after the detonation after receiving serious burns. Of the 20-day survivors, 1800 had been closer than 1000 meters to ground zero, 13.5% of whom had skin burns. A total of 5800 20-day survivors had been at distances of from 1500 to 2000 meters 18.1% of whom had skin burns. Of the 54, 600 survivors who had been at distances of from 4000 to 5000 meters, only 2.9% had skin burns. The data are presented in a form to allow estimates of casualties for situations resembling those at Nagasaki. From an analysis of the casualty data and from estimates made by Japanese doctors of the number of persons who survived immediately after the detonation, a casualty rate curve has been derived for the 20-day interval following the detonation.