Objective: Left ventricular hypertrophy (LVH) is the most common target organ damage in hypertension. However, the association of LVH and circadian rhythm of blood pressure (BP) is unknown. The objective of the present study was to explore the relationship between circadian rhythm of BP and LVH using ambulatory blood pressure monitoring (ABPM). Study design:A total of 325 untreated hypertensive patients were recruited. The patients were divided into two groups: hypertensive patients with LVH (n=121) and without LVH (n=204). Twenty-four-hour ABPM was performed in all the patients to collect the following parameters: 24-hour average systolic and diastolic pressure, daytime average systolic and diastolic pressure, nocturnal average systolic and diastolic pressure, and night to day BP ratio. The relationship between LVH and the various ABPM parameters was analyzed. Results:We found that the average nocturnal systolic blood pressure (SBP) in hypertensive patients with LVH was higher than that in hypertensive patients without LVH (145 ± 16.1 mmHg versus 136 ± 12.7 mmHg, P<0.05). The average night to day SBP ratio in hypertensive patients with LVH was also higher than that in hypertensive patients without LVH (0.93 ± 0.04 versus 0.86 ± 0.04, P<0.05). Multiple regression analysis indicate that the average nocturnal SBP and the night to day SBP ratio were associated with LVH (Odds ratio (OR) 1.67, 95% CI: 1.31-3.21; OR: 1.88, 95% CI: 1.56-3.78) by adjustment for traditional covariates. Conclusion:The average nocturnal SBP and the average night-day SBP ratio are independent risk factors of LVH in patients with hypertension.