Heart rate variability analysis has been widely used in clinical research as a noninvasive measurement of autonomic function. It has been found to be associated with post-myocardial infarction mortality, hypertension, sudden cardiac death, atherosclerosis, and diabetes. However, little epidemiologic research on HRV had been reported prior to this study in 1996. Almost no data were available on the population distribution of HRV, its correlates in populations, the factors associated with changes in HRV over time, or on the cardiovascular sequelae of impaired autonomic function assessed by HRV obtained from population-based prospective studies.