The Seventh-Day Adventist population abstains from smoking and drinking; about 50% follow a lacto-ovo-vegetarian diet; and most avoid the use of coffee, tea, hot condiments, and spices. Existing data on cancer mortality in Seventh-Day Adventists clearly document mortality rates that are 50 to 70% of general population rates for most cancer sites that are unrelated to smoking and drinking. Several approaches to determining whether this reduced risk is due to the unique Seventh-Day Adventist life-style or selective factors related to who choses to become and remain a Seventh-Day Adventist are described. A comparison of the mortality experience of Seventh-Day Adventist and non-Seventh-Day Adventist physicians shows equal cancer mortality, which is consistent with the hypothesis that the apparent reduced risk of cancer death in all Adventists may be due to selective factors. However, the results of a small case-control study of colon and breast cancer among Adventists show statistically significant relative risks for colon cancer of 2.8 for past use of meat. For current food use, the significant relative risks are 2.3 for beef, 2.7 for lamb, and 2.1 for a combined group og highly saturated fat foods. This strongly suggests that the lacto-ovo-vegetarian diet may protect against colon cancer. However, the evidence linking diet to breast cancer is less clear. Because of the marked variability in dietary habits within the Seventh-Day Adventist population, they will be a productive group for further study of diet and cancer.