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Introduction: Nighttime blood pressure (BP) measured by ambulatory BP monitoring (ABPM) is correlated with hypertensive target organ damage (TOD). Several recent studies had described an association between nighttime BP measured by home BP monitoring (HBPM), but there were few data comparing the association between TOD and nighttime BP measured by ABPM or HBPM. Hypothesis: We assessed the hypothesis that individuals who have ≥120 mmHg nighttime systolic BP (SBP) assessed HBPM and ABPM would be more likely to have TOD. Methods: We analyzed the data of 1,005 J-HOP study participants whose nighttime BP was measured by HBPM and ABPM. Nighttime home BP was automatically measured at 2, 3 and 4 a.m. by timer equipped device (HEM-5001, Omron Healthcare). Nighttime home BP was measured as many night as possible in 2 week study duration and mean of all values was used for the analysis. ABPM was measured every 30 min for 24 hour. Nighttime ambulatory SBP was calculated according to patient's diary. We classified the participants into four groups according to their nighttime SBP (mmHg): group 1, HBPM and ABPM <120; group 2, HBPM <120 and ABPM ≥120; group 3, HBPM ≥120 and ABPM <120; group 4, HBPM and ABPM ≥120. TOD as indicated by brachial ankle pulse wave velocity (baPWV), left ventricular mass index (LVMI) and carotid intima media thickness (IMT) were compared among 4 groups. Results: Mean age was 63±11 years; 49.9% were male. Nighttime home BP was measured 8.9±4.3 nights. For groups 1, 2, 3 and 4, respectively, baPWV were 1529±293, 1536±265, 1616±255 and 1710±322 cm/sec, LVMI were 91±23, 94±22, 101±26 and 101±28 g/m2, and IMT were 0.73±0.14, 0.72±0.18, 0.79±0.15 and 0.80±0.19 mm. After age, gender and clinic SBP were adjusted, baPWV, LVMI and IMT of groups 3 and 4 were significant higher than of group 1. In multivariate analyses, nighttime home SBP was an independent predictor of all TOD but nighttime ambulatory SBP was an independent predictor of only baPWV. Conclusions: Nighttime home SBP was more closely associated with TOD than nighttime ambulatory SBP. [ABSTRACT FROM AUTHOR]