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Chemoprevention provides an opportunity to complement screening for the prevention of colorectal neoplasia. Findings from prospective randomized trials often conflict with those of observational studies. This review discusses some of the possible reasons based on recent clinical trials.A recent prospective randomized trial demonstrates that folic acid supplementation in patients with a previous history of colorectal adenomas does not reduce future colorectal adenoma risk, and may possibly increase the risk of colorectal neoplasia.The results of prospective randomized human trials of chemopreventive agents have in many cases been less impressive or have conflicted with the results of observational studies. Issues to be considered are the timing of the intervention during multistep carcinogenesis, baseline levels in a given individual or population, the complexity of dietary interactions, dose–response effects, and the duration of study.