DISEASE CHARACTERISTICS: Colonic adenoma(s) of at least 10 mm completely resected within 18 months prior to randomization or suspected adenomatous polyp(s) Colonoscopy with submission of at least 7 rectosigmoid biopsies required at entry No benign hyperplastic polyps or polyps less than 10 mm to be eligible for treatment No polyposis coli (i.e., more than 100 polyps in colon) No history of invasive colorectal cancer