An insufficient decrease in nocturnal blood pressure (BP) is a known factor in cardiovascular mortality. We aimed to determine whether autonomic nervous system (ANS) activity and its change over 2 years were associated with a shift to non-dipper status, independently of initial BP, in a general elderly population. From participants in the PROOF study, 600 subjects untreated for hypertension were selected (age at baseline: 65 years, men: 41.5%). Dipper/non-dipper status was defined using repeated measures of 24-h ambulatory BP at baseline and 2 years later. ANS activity was evaluated on the basis of 24-h heart rate variability at both examinations. Among the 454 dipper subjects at baseline, 26.2% became non-dippers. Multivariate analysis showed that a +1 between-subject s.d. increase in the very low frequency at baseline was associated with a decreased odds ratio for the shift to non-dipper status 2 years later (OR¼0.61 [0.41-0.91], P¼0.02). The within-subject change between the two measurements of day and night systolic BP and day diastolic BP also contributed significantly to the risk of shift to non-dipper status. Our results suggest that impaired ANS activity precedes an insufficient decrease in nocturnal BP independent of hypertension status. Keywords: ambulatory blood pressure; heart rate variability; non-dipper; sympathetic nervous activity INTRODUCTION An insufficient decrease in nocturnal blood pressure (BP) (nondipping) is recognized as a marker of end-organ damage and a predictor of cardiovascular and all-cause mortality, independently of hypertension status. 1-3 Our understanding of the pathophysiologic factors that may be involved in the mechanisms of the circadian variation of BP and the non-dipping pattern remains incomplete. Among the suspected determinants, autonomic nervous system (ANS) activity is shown to be associated with arterial BP level and involved in the control of the circadian variation of BP. 4,5 Previous cross-sectional studies have shown that ANS activity, evaluated by heart rate variability (HRV), was reduced in non-dipper, untreated hypertensive subjects. 6,7 A possible explanation of the nocturnal decrease in BP is that it results from a reduction of sympathetic nervous activity and impaired circadian rhythm in sympatho-vagal balance during sleep. 8,9 We aimed to assess whether ANS activity indices and their changes over 2 years, as evaluated by 24-h HRV, were associated with a shift to non-dipper status among subjects identified as dippers at study entry in a general elderly population not treated for hypertension.