The antihypertensive effects of a telmisartan 80 mg/hydrochlorothiazide (HCTZ) 12.5 mg fixed-dose combination and telmisartan 80 mg monotherapy were compared in patients with a history of mild-to-moderate essential hypertension and inadequate BP control (DBP > or = 90 mm Hg) following 8 weeks of telmisartan monotherapy. At the end of this period, 491 patients (62.9% men; mean age 55.3 years) whose DBP was > or = 90 mm Hg were double-blind randomised to once-daily telmisartan 80 mg/HCTZ 12.5 mg (n = 246) or telmisartan 80 mg (n = 245). Trough (24 h post-dose) clinic BP was measured after 4 and 8 weeks of double-blind therapy. At the end of double-blind treatment, patients receiving telmisartan 80 mg/HCTZ 12.5 mg had significant additional decrements in clinic SBP/DBP over telmisartan 80 mg of -5.7/-3.1 mm Hg (P < 0.01). Most of the additional effect occurred during the first 4 weeks of treatment. The proportion of patients with normalised BP (SBP < 140 mm Hg and DBP < 90 mm Hg) was significantly greater in the telmisartan 80 mg/HCTZ 12.5 mg group than the telmisartan 80 mg group (41.5%vs 26.1%;P < 0.05). Both treatments were well tolerated. The incidence of adverse events was similar except for diarrhoea, which occurred more frequently in the telmisartan 80 mg/HCTZ 12.5 mg group, and oedema, which occurred more frequently in the telmisartan group. Our results indicate that a telmisartan 80 mg/HCTZ 12.5 mg fixed-dose combination confers significant additional BP reductions compared with continuation of telmisartan monotherapy in non-responders.