To investigate the association of autonomic nervous system dysregulation with blood pressure variability. Of 2370 participants in the X-CELLENT study, 577 patients (59.0±10.2 years) were randomly selected to participate in an ambulatory blood pressure monitoring ancillary study. We proposed a novel autonomic nervous system regulation index termed dSBP/dHR, which was defined as the steepness of the slope of the relationship between 24h systolic blood pressure and heart rate for each participant. Within-subject standard deviation of systolic blood pressure, weighted for the time interval between consecutive validated readings from 24h ambulatory blood pressure monitoring, was used to evaluate blood pressure variability. When dSBP/dHR was divided into tertiles, from tertile 1 to tertile 3, we observed a progressive increase in daytime systolic blood pressure, a progressive decrease in nighttime systolic blood pressure, and consequently a progressive increase in day-night systolic blood pressure gradient (P<0.001). On the contrary, standard deviation of both daytime and nighttime systolic blood pressure were consistently and significantly increased from tertile 1 to tertile 3 (P<0.01). Both before and after adjustment for age, gender and 24h mean blood pressure, all of these increasing or decreasing trends reached statistical significance (P<0.01). Furthermore, in our sensitivity analysis, when men and women were considered separately, the present finding remained unaltered. In summary, autonomic nervous system dysfunction was associated with the enlarged day-night systolic blood pressure gradient and more variable systolic blood pressure in 24 hours in patients with essential hypertension.