This prospective, double-blind, randomised, parallel-group, multicentre study assessed the adjunctive effect of telmisartan monotherapy versus placebo in controlling blood pressure during the last six hours of the 24-hour dosing period. After a two-week run-in phase, 375 patients with essential hypertension uncontrolled on existing therapy were randomised to either placebo or telmisartan (40 mg uptitrated to 80 mg after four weeks, if needed) for eight weeks. Ambulatory blood pressure monitoring (ABPM) was conducted at randomisation (baseline) and treatment end. The change from baseline in diastolic blood pressure (DBP) over the last six hours (primary endpoint) was significantly greater with telmisartan than placebo (adjusted mean treatment difference in favour of telmisartan: -3.7 mmHg, 95% confidence interval (CI) -5.5, -1.9 mmHg, p < or = 0.001, n = 350), as was the reduction in 24-hour DBP (adjusted mean treatment difference: -5.0 mmHg, 95% CI -6.5, -3.5 mmHg, p < or = 0.001). Telmisartan also reduced mean systolic blood pressure significantly more than placebo over the last six hours and the entire 24-hour dosing interval. Responder rates (ABPM DBP, seated DBP, and overall [seated SBP/DBP]) at 8 weeks were significantly higher with telmisartan than with placebo (p < or = 0.01). All treatments were well tolerated. When added to existing antihypertensive regimens, telmisartan offers additional effectiveness while maintaining placebo-like tolerability.