This randomised, double-blind study was designed to investigate the efficacy of a once-daily (OD) combination of the AT(1) receptor blocker, eprosartan 600 mg, and the thiazide diuretic, hydrochlorothiazide (HCTZ) 12.5 mg, in patients with mild to moderate hypertension (sitting diastolic blood pressure (sitDBP) greater than or equal to98 mm Hg and less than or equal to 114 mm Hg) not adequately controlled with eprosartan 600 mg OD. A total of 494 patients entered the open-label monotherapy run-in phase, which consisted of eprosartan 600 mg OD for 3 weeks. Patients who responded to monotherapy were not eligible to enter the randomised phase of the study and were withdrawn. The remaining 309 patients were then randomised to either eprosartan 600 mg plus HCTZ 12.5 mg OD or to continue on eprosartan 600 mg OD. In the eprosartan plus HCTZ combination group, both sitDBP and sitting systolic blood pressure (sitSBP) were significantly reduced compared with the eprosartan monotherapy group. In addition, the response rate was higher in the combination group compared with the monotherapy group. There were no significant effects on reduction of sitDBP due to gender, prior use of antihypertensives or baseline severity of hypertension. The tolerability profile for the combination group was similar to that for the monotherapy group. Headache was the most frequent adverse event in both treatment groups. The majority of adverse events were mild to moderate in intensity. In this study of patients who were unresponsive to eprosartan monotherapy for 3 weeks, a combination product of eprosartan 600 mg and HCTZ 12.5 mg was shown to be an effective and well tolerated treatment.