Data from the Utah Cancer Registry were used to compare cancer incidence in Mormons and non-Mormons in Utah for the period 1967--75. Church membership was identified for 97.8% of the 20,379 cases in Utah by a search of the central membership files of the Church of Jesus Christ of Latter-Day Saints (or Mormon Church). Sites associated with smoking (lung, larynx, pharynx, oral cavity, esophagus, and urinary bladder) showed an incidence in Mormons at about one-half that of non-Mormons. Rates of cancers of the breast, cervix, and ovary were low in Mormon women; the rate for cervical cancer was about one-half of that observed in non-Mormons. Cancers of the stomach, colon-rectum, and pancreas were about one-third lower in Mormons than in others who are not members of this religious group. Most of the differences seen in cancer incidence can be explained by Mormon teachings regarding sexual activity and alcohol and tobacco use, but some differences (e.g., colon and stomach) remain unexplained.