In this review, large-scale randomized intervention trials evaluating the effects of vitamin and mineral supplementation on cancer rates are summarized. The trials enrolled up to 30,000 adults who were followed for up to 12 years, and included assessments of multiple vitamin and mineral combinations in an area of China with limited micronutrient intake and one of the world's highest cancer rates; and of beta carotene, vitamin E, or selenium in several more well nourished western populations, some at very high risk of lung cancer. Results to date have been mixed. Significantly lower cancer mortality has been found among those supplemented with a combination of beta carotene, vitamin E, and selenium in the China trial and with selenium in the United States, but risks of lung cancer were increased in Finnish and American trials provided with high-dose beta carotene supplementation. In combination, the trials indicate that the relation between specific micronutrient intake and cancer risk is complex, but have provided information to target further research on the potential benefits of selenium, vitamin E, and combinations of vitamins and minerals.