In order to compare the antihypertensive effectiveness and tolerability of a standardised extract from Hibiscus sabdariffa with captopril, a controlled and randomised clinical trial was conducted. Patients with diagnosed hypertension and without antihypertensive treatment were included. The experimental procedure consisted of the administration of an infusion prepared with 10 g of dry calyx from H. sabdariffa on 0.5l water (9.6 mg anthocyanins content) daily before breakfast or captopril, 25 mg twice a day, for 4 weeks. The outcome variables were tolerability, blood pressure and, in the experimental group, urinary electrolytes modification. The analysis included 39 and 36 patients from the experimental and control groups, respectively. The results showed that H. sabdariffa was able to decrease the systolic blood pressure (BP) from 139.05 to 123.73 mm Hg and the diastolic BP from 90.81 to 79.52 mm Hg. At the end of the study, there were no significant differences between the BP detected in both treatment groups. The rates of therapeutic effectiveness were 0.7895 and 0.8438 with H. sabdariffa and captopril, respectively, whilst the tolerability was 100% for both treatments. A natriuretic effect was observed with the experimental treatment.