Selenium and vitamin E micronutrients have been advocated for the prevention of colorectal cancer. Colorectal adenoma occurrence was used as a surrogate for colorectal cancer in an ancillary study to the Selenium and Vitamin E Cancer Prevention Trial (SELECT) for prostate cancer prevention. The primary objective was to measure the effect of selenium (as selenomethionine) on colorectal adenomas occurrence, with the effect of vitamin E (as α-tocopherol) supplementation on colorectal adenoma occurrence considered as a secondary objective. Participants who underwent lower endoscopy while in SELECT were identified from a subgroup of the 35,533 men randomized in the trial. Adenoma occurrence was ascertained from the endoscopy and pathology reports for these procedures. Relative Risk (RR) estimates and 95% confidence intervals (CI) of adenoma occurrence were generated comparing those randomized to selenium versus placebo and to vitamin E versus placebo based on the full factorial design. Evaluable endoscopy information was obtained for 6,546 participants, of whom 2,286 had 1+ adenomas. Apart from 21 flexible sigmoidoscopies, all the procedures yielding adenomas were colonoscopies. Adenomas occurred in 34.2% and 35.7%, respectively, of participants whose intervention included or did not include selenium. Compared with placebo, the RR for adenoma occurrence in participants randomized to selenium was 0.96 (95% CI, 0.90–1.02; P = 0.194). Vitamin E did not affect adenoma occurrence compared with placebo (RR = 1.03; 95% CI, 0.96–1.10; P = 0.38). Neither selenium nor vitamin E supplementation can be recommended for colorectal adenoma prevention. Cancer Prev Res; 10(1); 45–54. ©2016 AACR.