Religiously active Mormons in California are a nonsmoking population with unusually low risk for cancer. This finding is based on the results of our 1979 questionnaire survey of life-style and the 8-year (1980-1987) follow-up of mortality among 5,231 Mormon high priests and 4,613 wives 25-99 years of age. Our study, which is the first prospective cohort study of Mormons, shows low standardized mortality ratios (SMRs) for this population, relative to those for whites in the general population in the United States, which are defined as 100. The SMRs for males are 47 for all cancers, 52 for cardiovascular diseases, and 47 for all causes; the SMRs for females are 72 for all cancers, 64 for cardiovascular diseases, and 66 for all causes. For middle-aged high priests adhering to three health practices (never smoking cigarettes, engaging in regular physical activity, and getting proper sleep), the SMRs are 34 for all cancers, 14 for cardiovascular diseases, and 22 for all causes. These results have been largely replicated in an active Mormon-like subgroup (white nonsmokers attending church weekly) from a representative sample of residents of Alameda County, CA. Our findings confirm and expand on previous descriptive studies of Mormons and demonstrate how these results can be generalized.