The present study describes the results from the 10-year follow-up data of a prospective epidemiological study for hypertension and cardiovascular diseases in two communities of rural agricultural districts in Hokkaido, Japan. The number of incidences of cerebrovascular accidents (CVAs) in persons who were normotensive, borderline hypertensive (BHT), untreated hypertensive (HT), well-controlled HT [blood pressure (BP) less than 150/90 mm Hg], and poorly controlled HT (BP greater than or equal to 150/90 mm Hg) were 0.46, 3.24, 4.17, 3.49, and 12.76 per 1,000 person-years. respectively: CVAs were markedly high in poorly controlled HT persons. The winter-summer mean BP differences in the first year were significantly and positively correlated with the differences in mean BP between the tenth and the first year, and were significantly higher in the progression to hypertension group than in the nonprogression group in both towns. Multivariate analysis indicated that the winter-summer mean BP difference was a significant variable for indication of progression to hypertension. From these results, we concluded that (a) good control of hypertension could considerably prevent CVA, (b) cold environment may contribute to the progression to hypertension, and (c) winter-summer variation in BP may predict the future course of BP.