In a double-blind randomized study, hydralazine (n = 59) or the new dihydropyridine calcium antagonist felodipine (n = 61) was added to previous treatment with beta-adrenoceptor blocking agents in a group of 120 patients with essential hypertension. Active treatment with either hydralazine or felodipine was given for 8 weeks after a 4-week placebo run-in period, at the end of which all patients had supine diastolic blood pressures greater than 95 mm Hg. Assessment of the results according to the intention to treat principle showed that felodipine was significantly more effective than hydralazine at the doses employed, reducing systolic blood pressure 10-19 mm Hg more than hydralazine and reducing diastolic blood pressure 5-11 mm Hg more than hydralazine (95% confidence intervals). The number of patients complaining of side effects, the number of complaints, and the number of patients that had to be withdrawn from treatment were numerically higher during treatment with hydralazine than with felodipine, but these differences were not statistically significant. Against this background it is concluded that felodipine is superior to hydralazine when added to an antihypertensive regimen consisting of beta-adrenoceptor blocking agents.