Aged garlic extract (AGE) is a material that has the possibility of a cancer-preventive effect according to epidemiologic and animal studies. In order to confirm the effects of AGE on colorectal adenomas, we conducted a double-blinded randomized study using high-AGE (AGE 2.4 ml/day) and low-AGE (AGE 0.16 ml/day) doses 1 groups. Fifty-one patients who were diagnosed as having colorectal adenomas by colonoscopy were randomly assigned to the high-AGE and low-AGE groups. The number and size of adenomas before intake (0 month) and 6 and 12 months after intake were measured using colonoscopy. In 37 patients chosen as efficacy evaluated subjects, 47.4% (9/19) in the high-AGE and 66.7% (12/18) in the low-AGE group had at least one new adenoma for the first and second interval (0 to 12 months after intake), and its relative risk was 0.71. The decrease rate of at least one adenoma was 50.0% (7/14) in the high-AGE group for the second interval (6 to 12 months after intake), whereas there was no decrease in subjects in the low-AGE group (p=0.02). The difference from the base-line for total size of adenomas increased in the low-AGE group, whereas an increase in the high-AGE group was suppressed for the second interval (p=0.04). The difference from the base-line for the total size of adenomas in subjects who had adenomas on the base-line increased in the low-AGE group and decreased in the high-AGE group for the second interval (p=0.03). The results of this study suggest the possibility of preventive and therapeutic effects of AGE on colorectal adenomas, though it is necessary to investigate these in larger-scale and longer-term trials.