In the 13.5 years following the detonation of the atomic bomb, 95 cases of leukemia have been observed in the Nagasaki survivors. This increase is highly significant statistically. The increased leukemia risk apparently started 1.5 to 2.5 years following radiation exposure, and has lasted through 1958. Acute leukemias of all types and chronic granulocytic leukemia are increased, (with the possible exception of the Schilling type of acute monocytic leukemia). Males in general, and individuals in the younger ages (0 to 09), are apparently most sensitive. The risk of radiation induction of leukemia is related to the size of the dose. The shape of the curve does not differ greatly from a linear model, but is consistent with a variety of hypotheses. The data in the low dose region are too limited to be of significance in evaluating the risk of low doses of radiation. The data suggest that high radiation doses may be associated with a decrease in the latent period to leukemia induction. 43 references, 2 figures, 31 tables.