OBJECTIVE: To investigate the risks of high and low blood pressure levels determined by ambulatory blood pressure monitoring and home blood pressure measurements. METHODS: A long-term prospective study of all-cause mortality and cardiovascular mortality has been conducted in Ohasama, Japan since 1987. RESULTS: The Cox proportional hazards model after adjusting for age and sex demonstrated that, during a 5-year follow-up period involving 893 of the subjects aged 50 years or more in this cohort, those with the lowest quintile of ambulatory blood pressure levels exhibited a significantly higher hazard ratio for cardiovascular and for all-cause mortality. During the same follow-up period, of 1226 subjects aged 50 years and more, those with the lowest and highest quintiles of home blood pressure levels demonstrated a significantly higher hazard ratio with respect to cardiovascular and all-cause mortality (i.e., a J-shaped relationship). CONCLUSION: These results suggest that there is a significant risk associated with low blood pressure levels, which can be determined by ambulatory and home blood pressure measurements, but not by casual blood pressure measurements.