Although the epidemiological study on hypertension should be started by the investigation of blood pressure levels of population groups, i, t was started by the study on cerebrovascular disease in Japan, because the presence of hypertension is considered to play a relatively important role, as an underlying disease, in the occurrence of cerebrovascular disease. On the other hand, the epidemiological study was started by the study on heart disease in Europe and the United States of America. At all events, it has been proven by the studies carried out later that hypertension is closely related to cerebrovascular disease in Japan. The blood pressure level of Japanese varies with population groups within the country and is not uniform, but the authors have been interested in comparison of blood pressure levels between Japan and foreign countries. When the authors were engaged in the Asahan Health Improvement Project (Proyek Pengenbangan Kesehatan Asahan) conducted on North Sumatra in Indonesia, they had an opportunity to determine blood pressure of inhabitants in Medan City in cooperation with medical students of the North Sumatra University. This investigation was done together with a comprehensive health improvement action program by the Project.