Between 1950 and 1969 cancer mortality in white Utah residents was 22 per cent less than that in the entire United States population. The religion of 72 per cent of the State residents (Mormon) proscribes use of tobacco and alcohol. We therefore analyzed the 10,641 cases of cancer identified in Utah from 1966 to 1970 and compared the incidence found in Utah Mormons, in Utah non-Mormons, and in a national survey. Comparison of Utah Mormons with non-Mormons showed that Mormons had a lower incidence of all cancers associated with cigarette smoking (P less than 0.00001). Mormon females had a low incidence of cancer of the breast (P = 0.008), uterine cervix (P less than 0.00001), and ovary (P = 0.04); Mormon males had a lower incidence of stomach cancers (P = 0.003). These findings addevidence to the association between cigarette smoking and certain cancers, but leave unexplained the significant differences between Mormons and non-Mormons for incidence of cancer of the breast, cervix, prostate and nervous system.