Oral telmisartan/hydrochlorothiazide (HCTZ) combines two antihypertensive agents, a selective angiotensin II receptor antagonist with a long half-life and once-daily administration, and a thiazide diuretic. In two large, 8-week, double-blind trials, patients with hypertension unresponsive to monotherapy who received combined telmisartan/HCTZ 80/12.5 or 40/12.5 mg/day, achieved significantly larger reductions in diastolic and systolic blood pressure (BP), than recipients of continued telmisartan monotherapy (p < 0.05 for all). Compliance with telmisartan/HCTZ 80/12.5 mg/day was 98.9%. In patients with hypertension, telmisartan/HCTZ resulted in similar BP reductions to oral enalapril/HCTZ and atenolol/HCTZ in 26-week double-blind trials and greater reductions than oral losartan/HCTZ 50/12.5 mg/day in a 6-week randomised open-label trial (p < 0.001). Up to one-third of patients with hypertension initially responsive to telmisartan 40 or 80 mg/day in a 4-year study required the eventual addition of HCTZ 12.5 or 25 mg/day and/or another agent to maintain BP control. BP was controlled in about 75% of these by adding only HCTZ. In clinical trials of up to 4 years, including elderly patients, telmisartan/HCTZ had similar tolerability to placebo, with few reports of hypokalaemia. Most adverse events were mild to moderate.