The study was ancillary to ARIC, a population-based, longitudinal study of cardiovascular and pulmonary diseases. The baseline exam was completed in 1987 to 1988, followed by yearly contacts and re-examinations every three years. The present study built on the data collected by the ARIC investigators by retrieving and processing beat-to-beat heart rate data collected during the baseline exam. Five minutes of beat-to-beat heart rate data were obtained from the ARIC cohort participants during their third follow-up visit (Visit) 4 in 1996 through 1998. Time and frequency domain HRV indices were derived for an assessment of autonomic function. The following HRV indices were computed both for the baseline and the nine-year follow-up exam (1996 through 1998) on the 13,000 members of the ARIC cohort: time domain indices; mean heart rate, minimum and maximum heart rate, standard deviation of all normal R-R intervals, the coefficient of variation of all normal R-R intervals, root mean square of the differences of successive R-R intervals, and the proportion of adjacent R-R intervals. Frequency domain indices were also computed, including high frequency power, low frequency power, and the high/low frequency power ratio.