On the basis of Church records, detailed cancer and total death rates were determined for an average of 360,000 California Mormons during 1968--75, for an average of 700,000 Utah Mormons during 1970 and 1975, and for a subgroup of active Mormon males known as High Priests and Seventies. For cancer as a whole, the standardized mortality ratio was 68% for all California Mormon males, 83% for all California females, and 50% for active Mormon males in California and Utah compared with 1970 U.S. whites. Age-specific and age-adjusted total mortality rates were substantially lower in Mormons than in 1970 U.S. whites, with the greatest differences occurring between 35 and 65 years of age, where the rates for active Mormon males were reduced by more than 60%. Methodologic issues and sources of error were discussed, and the overall quality of the data was good. Some health-related characteristics of Mormons are also summarized.