Antihypertensive efficacy, effects on left ventricular mass index (LVMI) and tolerability of telmisartan, an angiotensin II receptor blocker, were compared with those of hydrochlorothiazide (HCTZ). Adult patients with mild-to-moderate hypertension and an optimal acoustic window by two-dimensional echocardiography were randomised at baseline to 12 months' double-blind, once-daily treatment with telmisartan 80 mg or HCTZ 25 mg. Two-dimensional echocardiography and freehand precordial three-dimensional echocardiography and 24-h ambulatory blood pressure monitoring were performed at baseline and after treatment. Of the 41 telmisartan group patients and 28 HCTZ group patients, 40 and 25, respectively, completed the study. Following treatment, 24-h mean SBP (telmisartan 157 7 11 vs 133 7 7 mmHg, Po0.001; HCTZ 154 7 10 vs 144 7 11 mmHg, Po0.003) and DBP (telmisartan 96 7 6 vs 83 7 5 mmHg, Po0.001; HCTZ 95 7 7 vs 87 7 8 mmHg, Po0.003) were significantly reduced. Telmisartan produced significantly greater 24-h mean SBP and DBP reductions than HCTZ (Po0.001). LVMI was significantly reduced by telmisartan (141 7 16 vs 125 7 19 g/m 2 , Po0.001), but not by HCTZ (139 7 20 vs 135 7 22 g/m 2 ). Incidences of adverse events in both the treatment groups were low; two cases of hypokalaemia occurred with HCTZ. In conclusion, telmisartan 80 mg was well tolerated and significantly reduced SBP, DBP and LVMI after 12 months' treatment compared with HCTZ.