The rapidly increasing incidence of colorectal cancer in Japan poses a great challenge to researchers to develop preventive strategies against this disease. Thus far, several clinical trials for this purpose have been planned in Japanese subjects; some have been completed and documented while others are still ongoing. Also, the Ministry of Health, Labour and Welfare of Japan recognizes the significance of cancer prevention studies, especially against colorectal cancer, including it as one of the pillars in the "Third Research Project on General Strategies against Cancer" and funding several large-scale projects. Among them are two chemoprevention studies currently being performed: in patients with previous sporadic colorectal tumors (J-CAPP study) and in patients with familial adenomatous polyposis (J-FAPP study II). Both are double-blind randomized controlled trials with low-dose aspirin (100 mg/day), which is generally considered to be safe for long-term use. This article outlines relevant past clinical data and gives a brief introduction to these two studies.