Objective: To investigate the morning blood pressure surge (MBPS) in Chinese patients with mild to moderate essential hypertension treated with long-term administration of olmesartan, an angiotensin II receptor antagonist according to ambulatory blood pressure monitoring (ABPM). Materials and Methods: In a multi-center, prospective study, we investigated the long-term efficacy of olmesartan by ABPM in 18-75 years-old Chinese patients with mild to moderate hypertension (clinic diastolic blood pressure [DBP] 90-109 mm Hg and systolic blood pressure [SBP] < 180 mmHg). After a 1 week placebo run-in, 87 patients were treated with olmesartan 20 mg once daily in the morning for 24 weeks. Ambulatory blood pressure monitoring was conducted at baseline and at the end of 24 weeks. At baseline, patients with an MBPS >= 23 mmHg were classified as the MBPS group (n = 41), and all other patients were classified as the non-MBPS group (n = 46). Results: The mean systolic and diastolic blood pressures (SBP/DBP) over 24 hours were reduced from 141.78 +/- 12.8/91.17 +/- 7.34 to 128.35 +/- 15.86/83.58 +/- 9.53 mmHg (p < 0.01). The mean blood pressure in the final 6 hours of the dosing interval dropped from 135.75 +/- 5.84/87.29 +/- 4.80 mmHg to 122.98 +/- 6.46/80.49 +/- 4.31 mmHg (p < 0.01). The MBPS for SBP/ DBP were reduced from 35.68 +/- 8.85/29.77 +/- 17.19 mmHg to 28.62 +/- 15.08/19.08 +/- 11.01 mmHg in the MBPS group (p < 0.05). The reductions in MBPS after treament with olmesartan were significantly different between the two groups (p < 0.01). Conclusions: Olmesartan effectively reduces blood pressure in patients with essential hypertension, and olmesartan especially reduces the MBPS in MBPS-prone patients.