Objective To assess the efficacy of carvedilol in treating essential hypertension by 24-h blood pressure monitoring Method Twenty patients with WHO stage I or II essential hypertension were enrolled in this study After a washout phase with placebo, patients received 4 weeks of treatment with carvedilol at a dose of 20 ～ 40 mg once daily Twenty-four hours ambulatory blood pressure was measured before and after treatment, and the trough-to-peak ratio (T/P ratio) was calculated Results Carvedilol induced a significant reduction in 24-h ambulatory blood pressure in the daytime The average reductions were 8 2 % in SBP and 8 5 % in DBP in daytime No significant reduction was seen in 24-h ambulatory blood pressure at night Morning blood pressure returned to the level before treatment The reduction of heart rate in daytime was also greater than that of patients at night Among the 20 cases, 9 cases had effective reduction in blood pressure, and the T/P ratio of SBP and DBP were 61 3 % and 39 8 %, respectively The T/P ratio did not meet the standard of FDA for antihypertensive drug with once-a-day administration Conclusion Carvedilol can control hypertension effectively in the daytime, but it induces no significant reduction in morning blood pressure suggesting that the dosage of 10～20 mg twice a day is optimal