We conducted a randomized, controlled clinical trial to test the efficacy of beta carotene and vitamins C and E in preventing colorectal adenoma, a precursor of invasive cancer.The lack of efficacy of these vitamins argues against the use of supplemental beta carotene and vitamins C and E to prevent colorectal cancer.the relative risk for beta carotene was 1.01 (95 percent confidence interval, 0.85 to 1.20); for vitamins C and E, it was 1.08 (95 percent confidence interval, 0.91 to 1.29).