The efficacy and acceptability of Rilmenidine (RIL) were assessed in a multicentre, controlled, double-blind trial versus hydrochlorothiazide (HCZ) in patients with mild to moderate hypertension. After an initial 4 week washout period on placebo, patients with diastolic BP of between 90 and 110 mmHg were administered in monotherapy either RIL 1 mg or HCZ 25 mg for four weeks (Day 0-Day 28). In the following four weeks (Day 28-Day 56), those with diastolic BP of over 90 mmHg were given, in association, the antihypertensive agent that they had not received initially. The other patients continued the trial with single daily doses of monotherapy. Two hundred and fourty four patients (48.4 +/- 0.6 years) with supine systolic and diastolic BP of 155.86 +/- 0.96 mmHg and 101.02 +/- 0.28 mmHg respectively were randomly allocated to two comparable treatment groups (RIL, n = 120 and HCZ, n = 124). The systolic and diastolic BP decreased significantly between Day 0 and Day 28, as much at Day 14 as at Day 28 in both treatment groups. The antihypertensive effect was comparable: at Day 28 the mean fall in systolic/diastolic BP was 16/10 mmHg in the RIL group and 15/9 mmHg in the HCZ group (NS). The clinical acceptability was equivalent in the two groups: secondary effects were responsible for one patient's withdrawal in each group and their incidence was 5% on average, 10% for the most common.(ABSTRACT TRUNCATED AT 250 WORDS)