This document contains two reports. The aim of the first investigation was to obtain information concerning all individuals in Hiroshima and Nagasaki having onset of symptoms of leukemia or dying of the disease since the atomic explosion in 1945. Results show that: (1) There is a significant increase in the incidence of leukemia in the exposed populations of Hiroshima and Nagasaki as compared with the non-exposed populations of the two cities; (2) there is a significant increase in the incidence of leukemia within the exposed population of Hiroshima and Nagasaki in subjects exposed at distances less than 2000 meters from the hypocenter; and (3) The concept that radiation from the atomic bomb explosions in Hiroshima and Nagasaki is a leukemogenic agent in man is supported. In the second report, 10 patients were used to study the early hematologic and preclinical phases of leukemia in atomic bomb survivors. Findings are presented. 23 references, 13 figures, 15 tables.