In this study we compared the antihypertensive efficacy and tolerability of captopril at 25 mg twice daily, hydrochlorothiazide (HCTZ), 12.5 mg twice daily and placebo in a multicentre, double-blind, randomized study that included 152 essential hypertensive patients (77 males, 75 females, 87 WHO stage I, 65 WHO stage II, aged 69 +/- 4 years, mean +/- s.d.). Supine and standing blood pressure were similarly reduced by captopril and HCTZ (P less than 0.01 for both compared with placebo). The heart rate did not change. Captopril (25-30 mg twice daily) and HCTZ (12.5 mg twice daily), alone or in combination, maintained their antihypertensive effect during a 24-week single-blind follow-up study. During the follow-up, diastolic blood pressure remained less than 100 mmHg in seven essential hypertensives on placebo, in 45 on captopril and in 25 on HCTZ. Side effects were observed in seven essential hypertensives during placebo (treatment withdrawn in two), in eight during HCTZ and in three during captopril. Serum potassium was reduced (P less than 0.05) and uric acid was increased (P less than 0.01) only during HCTZ. We conclude that captopril and HCTZ have similar antihypertensive efficacy in the elderly; however, captopril appears to be better tolerated.