A water-soluble extract from a cultured medium of Ganoderma lucidum mycelia (MAK) is one of the G. lucidum extracts that has been reported to show have exhibit cancer-preventive effects in the animal studies. To confirm cancer-preventive effects of MAK, we performed a no-treatment concurrent controlled trial on patients with colorectal adenomas. Patients who were determined to be carrying colorectal adenomas by colonoscopy were enrolled in this study. Patients in the MAK group took MAK (1.5 g/day) for 12 months. Follow-up colonoscopy was performed after 12 months, and the colonoscopists recorded the size, site and macroscopic type of all adenomas. Among 123 patients who enrolled in the MAK group, 96 eligible patients completed the trial. The 102 eligible patients in the no-treatment control group were selected randomly from our department's patients. The changes in the number of adenomas up to 12 months increased to 0.66 ± 0.10 (mean± SE) in the control group, while decreasing in the MAK group to -0.42 ± 0.10 (p<0.01). The total size of adenomas increased to 1.73 ± 0.28 mm in the control group and decreased to -1.40 ± 0.64 mm in the MAK group (p<0.01). The results suggest that MAK suppresses the development of colorectal adenomas - precancerous lesions of the large bowel.