Using information from the Supplementary Schedules of the 1950 National Census and from the JNIH-ABCC Life Span Study, cumulative person-years at risk in 1950 to 1960 were estimated by age ATB, sex, distance from hypocenter, radiation dose and symptoms for A-bomb survivors resident in Hiroshima and Nagasaki cities. The number of person-years at risk in 1951 to 1958 was estimated by applying the survivorship in each age group of the Adult Health Study sample during the period 1951 to 1958 to the number of survivors in 1950. To determine the number of person-years at risk from 1959 to 1960, the average yearly loss was evaluated for each exposure group for the period 1955 to 1958 in Hiroshima and for 1953 to 1958 in Nagasaki which was then applied to 1959 and 1960, respectively. The estimate of person-years among the nonexposed groups for this period was obtained from the above estimates, the total population of both cities, and the number of persons born after the A-bombing. Estimates by other associated factors were obtained by the same procedure. 20 references, 25 tables.