The purpose of our study was to evaluate the antihypertensive efficacy, tolerability and effects on left ventricular mass of losartan over 10 months in patients with essential hypertension. Losartan is a selective angiotensin II receptor-antagonist. The whole study comprised 89 hypertensive patients who were randomized, at baseline, to 10 months of double-blind once daily treatment with losartan 50 mg (L Group, n = 49, mean age 55 +/- 13 years) or hydrochlorothiazide 25 mg (HCTZ Group, n = 40, mean age 56 +/- 10 years). Routine hematology, blood chemistry and urinalysis were performed before and after 5 and 10 months; standard electrocardiography, ambulatory non invasive 24-hours blood pressure monitoring, M-mode echocardiography, psychometric test and quality of life evaluation were obtained from all the patients before and after 10 months. In patients non responding after 4 weeks, hydrochlorothiazide 25 mg or losartan 50 mg was added in the L Group and in the HCTZ Group, respectively (L-HCTZ Group). The results showed good tolerability and a significant mean systolic and diastolic blood pressure reduction in all groups (L Group from 157/96 +/- 9/7 to 137/85 +/- 9/5 mmHg, p < 0.001; HCTZ Group from 158/97 +/- 11/8 to 150/91 +/- 9/7 mmHg, p < 0.003; L-HCTZ Group from 159/98 +/- 9/5 to 141/88 +/- 6/4 mmHg, p < 0.001), although L and L-HCTZ treatment were more effective during 24 hours than HCTZ. Moreover, a remarkable reduction in left ventricular mass index was obtained after 10 months only in the L Group (from 138.4 +/- 26.2 to 127.2 +/- 23.1 g/m2, p < 0.04) and in the L-HCTZ Group (from 140 +/- 20.3 to 125.5 +/- 20.1 g/m2, p = 0.126). Finally, in patients treated with losartan the results of psychometric test significantly improved (L Group: p < 0.05; L-HCTZ Group: p < 0.05) and a positive remarkable change in the quality of life was observed (L Group: p < 0.05; L-HCTZ Group: p = 0.083). In conclusion, losartan in monotherapy or in association with hydrochlorothiazide, was well tolerated, respected the quality of life, and produced a significant and remarkable reduction in blood pressure and left ventricular mass in hypertensive patients.