© FICIAL USE ONLY 4. Mortality from cancer (excluding leukemia) in 1966-70 was sharply elevated among children under 10 years of age ATB exposed to more than 100 rads. Sensitivity to radiation carcinogenesis was a declining fanction of age at exposure when measured in terms of relative risk. 5. Study of lung cancer, specifically, is complicated by the fact that mortality ratios for this disease have been increasing very rapidly in these cities over the 20-year period studied. 6. Mortality from breast cancer was sharply elevated in 1965-70 among female survivors with doses exceeding 50 rads. The effect was strongest among those 10-19 years old ATB, and then declined with increasing age. No cases occurred among children under 10 ATB, but these children may not yet be old enough to show an effect. 7. Relative risks for the 200 + rad broup, compared with the 0-9 rad group, for the entire 20-year period varied widely for different diseases. Significant elevations were apparent for leukemia, other cancer, neoplasms of unspecified malignancy and possibly "other disease," but not significantly elevated for tuberculosis, stroke, or other cirulatory svstem disease. 8. There is some observed variation in the relative risks with reference to the specific sites for cancer. However, the variation, although large, is not statistically significant, because some of the numbers are small, With the additional data that will become available in future years the picture will become clarified as either the various risks draw together or, alternatively, the differences become statistically significant. 9, Leukemia mortality during the entire period 1950-70 was very nearly a linear function of total dose in rads. 10, Excluding leukemia, the relationship of cancer mortality to dose looks approximately linear, except for a sharp dip in the region 100-199 rads; the dip results wholly from an unexplained aberration in the Nagasaki data. Jl. In Hiroshima, the mortality ratios for cancer in the 200+ rad group increased constantly from 1950-54 to 1965-70, with an accelerated increase from 1960-64 to 1965-70. In Nagasaki, by contrast, the ratio was very high in 1950-54, declined during 1960-64, and then increased, in parallel with Hiroshima, in 1965-70. 12, With reference to the differences between Hiroshima and Nagasaki and a possible relationship to the RBE of neutrons as compared with gamma radiation for leukemia, the response in Hiroshima is very nearly a linear function of dose from the smallest to the largest dose levels. In Nagasaki, on the other hand, no response is visible below 109 rads, but above 100 rads the response appears linear and parallel to Hiroshima, OFFICIAL USE ONLY