Background: Blood pressure (BP) variability is estimated as the standard deviation (SD) of 24-h ambulatory BP1-3. The present study was performed to determine the effect of the mean day- and night- ambulatory BP values and standard deviations on carotid intima-media thickness (IMT) in treated hypertensive patients4-5. Methods: Carotid IMT and 24-h ambulatory BP were measured in 107 hypertensive patients with treatment (62.6 ± 14.8 years old; clinical systolic/diastolic BP: 144.0/82.3 mmHg). The mean value and SD of ambulatory BP were calculated for the entire 24-h period and separately for the day-time period and the night-time period. Carotid IMT was defined as the distance between the leading edge of the lumen-intima interface and the leading edge of the media-adventitia interface on the far wall, and the greatest IMT value in the bilateral longitudinal projections was recorded as the maximum IMT. Results: Univariate regression analyses revealed a significant association between carotid IMT values and the SD of ambulatory systolic/diastolic BP during night time (r = 0.296, p = 0.004; r = 0.205, p = 0.048, respectively). However, there were no association between carotid IMT and the 24-h systolic/diastolic BP, day-time systolic/diastolic BP, night-time systolic/diastolic BP, the SD of 24-h systolic/diastolic BP, and the SD of day-time systolic/diastolic BP. Multivariate regression analyses confirmed a significant correlation between carotid IMT and the SD of night-time systolic BP (β=0.232, p = 0.012). Conclusions: In hypertensive patients with treatment, a higher night-time ambulatory systolic BP variability had thicker carotid IMT. Only night-time ambulatory systolic BP variability may be involved in carotid IMT in treated hypertensive patients. References Pickering TG, James GDJ. Ambulatory blood pressure and prognosis. J Hypertens. 1994; 12(Suppl 8): S29–S33. Verdecchia P, Borgioni C, Ciucci A, Gattobigio R, Schillaci G, Sacchi N, et al. Prognostic significance of blood pressure variability in essential hypertension. Blood Press Monit. 1996; 1: 3–11. Kikuya M, Hozawa A, Ohokubo T, Tsuji I, Michimata M, Matsubara M, et al. Prognostic significance of blood pressure and heart rate variabilities: the Ohasama study. Hypertension. 2000; 36: 901–6. Hamada T, Murata T, Narita K, Takahashi T, Wada Y, Kimura H, Yoshida H, et.al. The clinical significance of abnormal diurnal blood pressure variation in healthy late middle-aged and older adults. Blood Press. 2008;17:134-40.