Atherosclerotic disease is the most prevalent cause of death worldwide. The ratio of coronary heart disease/cerebrovascular disease differs between Japan and Western countries and the incidence of hemorrhagic stroke and gastrointestinal bleeding is higher in Japan. Thus, the threshold for aspirin administration for primary prevention has been controversial in Japan. Much anecdotal data from Western countries and from Japan has implied that the threshold for administering aspirin to those with risk factors for coronary heart disease is higher than that recommended in Western countries, and that the potential candidates for primary prevention in Japan seem to be diabetic patients. The Japanese primary Prevention of atherosclerosis with Aspirin for Diabetes (JPAD) trial involving 2,530 patients with type 2 diabetes started in December 2002. Compared to other primary prevention trials, this trial offered an acceptable sample size, a standard aspirin dosage, and gender balance. Because stroke is the most significant component of all atherosclerotic diseases in Japan, the impact of primary prevention with aspirin on stroke should be understood. Thus, the JPAD trial should generate reliable data on primary prevention with aspirin for diabetic patients that would also be relevant to other countries.