[en] A study was made on the atomic bomb survivors in whom leukemia subsquently occurred in Hiroshima and Nagasaki. The incidence of leukemia averaged 30 per one million people 5--26 years after they had been exposed to less than one rad. Among the atomic bomb survivors who had been exposed to 1--99 rads, leukemia occurred within 5--10 years, which showed a slightly higher incidence than that among those who had been exposed to one rad. Among survivors exposed to more than 100 rads, leukemia occurred within 5--10 years on an average of 900 per one million, which was more than 30 times as high as that among those with one rad. Among those aged 15--29 years at the time of bombing, it was high 10--15 years afterward, but was low among those aged 30--44 years at the time of bombing. Among people over 45 years, it became high within 10--15 years, and was still high 15--26 years after bombing. It was suspected that it took 2--3 years after the bombing to develop. From 1950 to 1961, among survivors with exposure to more than 50 rads in Hiroshima, there was a higher incidence of leukemia than there was in Nagasaki. This suggests that neutron-rays have a stronger effect on the incidence of leukemia than have γ-rays. Survivors who had been exposed to neutron-rays in Hiroshima, often had acute or chronic leukemia. On the other hand, some of the survivors with exposure to γ-rays in Nagasaki had only acute leukemia. (Kanao, N.)