Effectiveness trials in hypertension enable the efficacy and safety of new drugs to be compared with previous therapy. Since these open-label trials could inadvertently be influenced by observer bias, this study has used ambulatory blood pressure monitoring (ABPM) to provide a rigorous blinded end point to validate the study conclusions. The study was performed in 675 patients with stage 1 or 2 hypertension despite receiving single-agent or fixed-dose combination therapy. After baseline ABPM, the previous treatment was replaced by telmisartan 40 mg daily; if control (office blood pressure <140/90 mm Hg) was not achieved in 2 weeks, the dose was increased to 80 mg, and if necessary, a fixed combination with hydrochlorothiazide 12.5 mg was used after a further 4 weeks. ABPM was repeated after 4 weeks on final therapy. Overall, 50% of patients finished on monotherapy and 50% on combination therapy. By office measurements, there was a decrease (mean +/- SEM) of 16.8+/-0.5/10.3+/-0.3 mm Hg (p<0.001) when telmisartan-based treatment replaced previous treatment; by ABPM, the decrease was 8.2+/-0.4/5.0+/-0.2 mm Hg (p<0.001). The decreases were significant for comparisons with each of the prior drug classes. A treatment algorithm based on the angiotensin receptor blocker, telmisartan, was confirmed by the blinded end point of ABPM as an efficacious alternative to other antihypertensive regimens in clinical practice.