[en] Cancer mortality among 40,498 Hiroshima and Nagasaki residents was examined in relation to cigarette smoking habits and estimated atomic bomb radiation exposure. Relative risk models that are either multiplicative or additive in the two exposures (smoking radiation) were emphasized. Most analyses were directed toward all nonhematologic cancer, stomach cancer, lung cancer, or digestive cancer other than stomach, for which there were, respectively, 1,725, 658, 281, and 338 deaths in the follow-up period of this study. Persons heavily exposed to both cigarette smoke and radiation were found to have significantly lower cancer mortality than multiplcative relative risk models would suggest for all nonhematologic cancer, stomach cancer, and digestive cancer other than stomach. Surprisingly, the relative risk function appeared not only to be submultiplicative for these cancer sites, but to be subadditive as well. The lung cancer relative risk function could not be distinguished from either a multiplicative or an additive form. The number of deaths was sufficient to permit some more detailed study of all nonhematologic cancer mortality: Relative risk functions appeared to be consistent between males and females though a paucity of heavy smoking females limits the precision of this comparison. (author)