The clinical efficacy and acceptability of once-daily perindopril (4 to 8 mg) monotherapy and in combination with hydrochlorothiazide (50 mg/day) was studied in mild to moderate stable essential hypertensive patients in 4 centres in Thailand. After 2-4 weeks of placebo run-in period, patients received active treatment for 3 months starting with 4 mg perindopril once daily. Dose titration was at second and third month of active treatment if the supine DBP was > 90 mmHg. The dose was doubled and if necessary, 50 mg/day hydrochlorothiazide was added in the last month. The results in 95 patients showed that the mean reduction in supine SBP/DBP at 1, 2 and 3 months of treatment was 10.3/8.0, 13.2/8.7 and 19.1/13.7 mmHg respectively. At the end of the study, 80 per cent of the patients showed normalisation of the supine diastolic blood pressure (supine DBP < or = 90 mmHg) with 30 per cent receiving combined therapy of perindopril and hydrochlorothiazide. There was no significant change in routine haematology or serum biochemistry except for slight increase of potassium levels in patients receiving 8 mg perindopril monotherapy. The incidence of side effects and withdrawal from treatment were quite low. Cough was the major side effect reported comprising 13.6 per cent with only 1 case withdrawn. The study confirms the previous studies that perindopril had satisfactory antihypertensive efficacy and acceptability profiles.