In this study, efficacy of the angiotensin II type 1 receptor blocker telmisartan given as monotherapy was compared with that of perindopril monotherapy in patients with mild-to-moderate hypertension. After a 2-week, single-blind, placebo run-in period, 60 patients were randomised to double-blind, once-daily treatment with telmisartan 80 mg or perindopril 4 mg for 6 weeks. Clinic and ambulatory blood pressure measurements and clinical laboratory evaluation were performed at the end of the placebo run-in and active treatment phases. Both telmisartan and perindopril significantly (p < 0.0001) reduced clinic systolic blood pressure (SBP) and diastolic blood pressure (DBP) compared with baseline values. Also, both drugs significantly (p < 0.0001) reduced 24-h mean ambulatory SBP and DBP compared with baseline. Comparison of the mean hourly antihypertensive activities showed that the reduction in mean ambulatory DBP for the last 8 h of the dosing interval was significantly greater (p < 0.05) in telmisartan-treated patients. A 24-h mean DBP of <85 mmHg was observed in 66.6% of the telmisartan-treated patients but in only 46.6% of the perindopril-treated patients (p < 0.05). It is concluded that telmisartan and perindopril both produce significant reductions in clinic SBP and DBP, but the mean reduction in ambulatory DBP during the last 8 h of the dosing interval is greater in patients treated with telmisartan.