In 1947 President Harry Truman authorized the National Academy of Sciences to develop studies to document the long-term health effects of exposures from the atomic bombings of Hiroshima and Nagasaki. The resulting organization, originally called the Atomic Bomb Casualty Commission (ABCC), but reorganized in 1975 as RERF with joint Japanese and American support, began the systematic follow-up in 1950 of the mortality experience of a group of 120,000 persons, both males and females of all ages, known as the Life Span Study (LSS). Based on national census information, this included as many as possible of those who were within 2.5 km of the hypocenter (the ground location directly beneath the bomb) and a random subsample, matched on age and sex, of the much larger numbers who had been between 2.5 and 10 km from the hypocenter. It also included about 26,000 who were not in the cities at the time of the bombing but resided there as of 1950. In 1958, a biennial systematic clinical examination program (Adult Health Study; AHS) was begun for a subsample of about 20,000 members of the LSS. Subsequently a sample of about 77,000 offspring of atomic bomb survivors, born in 1946-1984, was formed for mortality follow-up. Cancer registries were formed in Hiroshima and Nagasaki in 1958, the first such registries in Japan, to document the incidence of cancer among atomic bomb survivors and their offspring. The LSS, AHS and offspring studies continue to this day, and worldwide have been the primary basis for estimating the health risks from ionizing radiation exposure.