An open-label clinical trail comparing the effectiveness of the combination of methyldopa with hydrochlorothiazide in low doses with that of hydrochlorothiazide therapy alone was conducted in 21 elderly patients with isolated systolic hypertension. The combination therapy reduced the systolic blood pressure to levels markedly lower than those measured at baseline and after two weeks of hydrochlorothiazide therapy. Standing blood pressure fell from a mean of 166/90 mmHg at baseline to a mean of 164/88 mmHg after two weeks of hydrochlorothiazide treatment and to a mean of 132/80 mmHg after 16 weeks of treatment with hydrochlorothiazide and methyldopa. Side effects were mild, and there were no significant changes in laboratory findings.