The incidence of leukemia is higher among those closely exposed than among those more remote from the hypocenter. This increase was first manifest approximately three years following exposure. It apparently reached its peak between the years 1950 and 1952. Thereafter, the incidence has been diminishing, but 13 years after exposure it is still higher than would be expected in the general population. The type of leukemia most increased in incidence is the chronic granulocytic variety. No apparent difference in the natural history of the specific types of leukemia in exposed and nonexposed Japanese has been observed. Preliminary analyses of the data show a linear relation between dose of radiation and incidence of leukemia above 50 to 100 rad. Below this dose, the shape of the curve is not certain.