OUTLINE: This is a randomized, double-blind, placebo-controlled, multicenter study. Patients are stratified according to participating center and age (50-70 vs 30-49 and over 70). Patients with adenomatous polyps at least 10 mm at colonoscopy are randomized to 1 of 2 treatment arms. Patients found to have no neoplastic, hyperplastic, or inflammatory polyps at colonoscopy are assigned to a control (untreated) group and complete laboratory studies and a baseline food frequency questionnaire only. Arm I: Patients receive folic acid daily. Arm II: Patients receive placebo daily. Treatment continues in both arms for 1 year in the absence of unacceptable toxicity or disease progression. Patients are followed at 1 year.