The once-daily fixed combination of losartan 100 mg/hydrochlorothiazide 25 mg was evaluated for safety and efficacy in a multicenter open study by using 24-h ambulatory blood pressure monitoring in untreated patients with moderate-to-severe essential hypertension or patients with uncontrolled hypertension despite treatment with monotherapy or low-dose combination. After a 2-week washout period, 41 patients (22 men, 19 women) aged 34-74 years, showing a mean daytime blood pressure > 135/85 mm Hg, were treated with losartan 100 mg/hydrochlorothiazide 25 for 8 weeks. Ambulatory blood pressure was monitored at the end of the washout period and during the last week of treatment. A significant reduction in the average values of clinic blood pressure (from 169.9 +/- 13.5 mm Hg to 139.5 +/- 15.6 mm Hg, p < 0.001 for systolic blood pressure [SBP]; and from 102.2 +/- 7.1 mm Hg to 85.1 +/- 9.5 mm Hg, p < 0.001 for diastolic blood pressure [DBP]) was observed after treatment in the whole group of 41 patients. Likewise, average values of both 24-h SBP and 24-h DBP were significantly reduced (from 145.7 +/- 13.1 mm Hg to 128.3 +/- 14.6 mm Hg, p < 0.001 for 24-h SBP; and from 90.3 +/- 7.3 mm Hg to 79.2 +/- 8.6 mm Hg, p < 0.001 for 24-h DBP). The average lowering at peak was 20.2 +/- 11.8 mm Hg for 24-h SBP and 12.1 +/- 7.4 mm Hg for 24-h DBP, whereas the lowering at trough was 17.8 +/- 12.0 mm Hg and 10.4 +/- 8.1 mm Hg, respectively. The trough-to-peak ratio (T/P) was 0.88 for SBP and 0.86 for DBP, and the smoothness index was 7.36 for SBP and 6.37 for DBP. The response rate was 87.8% (blood pressure lowering > 5 mm Hg of either 24-h SBP or 24-h DBP average values). Among responders, T/P ratio was 0.89 for SBP and 0.87 for DBP, and the smoothness index was 8.09 for SBP and 7.15 for DBP. No side effects or changes in metabolic parameters were observed. The fixed combination of losartan 100 mg/hydrochlorothiazide 25 was very effective and well tolerated.