We compared amlodipine, a dihydropyridine calcium antagonist, to placebo as add-on therapy to hydrochlorothiazide in 91 hypertensive patients inadequately controlled on hydrochlorothiazide (50 mg/d for four weeks). This was a double-blind, randomized, multicenter, parallel group-trial; 45 patients received placebo and 46 received amlodipine in doses of 2.5 to 10 mg qd (mean 9 mg/d). Supine blood pressure systolic/diastolic, mean +/- SE mm Hg) 24-hour postdose was significantly reduced by 14.2 +/- 2.3/11.7 +/- 1, compared to placebo, 4.5 +/- 2.7/5 +/- 1.2. Standing blood pressure was similarly reduced: amlodipine by 14 +/- 2.7/12.5 +/- 1.2; placebo by 3 +/- 2.1/5.8 +/- 1.2. This reduction in blood pressure was attained without any significant changes in pulse rate, EKG, and serum lipids (triglycerides were reduced in the amlodipine group by 42.9 mg/dL, P = .023). Only two patients had side effects requiring discontinuation from the study (both in the amlodipine group). Side effects occurred in 27 amlodipine-treated patients (11 with peripheral edema) and 18 patients in the placebo (three with peripheral edema) group. Investigator's assessment of therapeutic effect and tolerability, and the percent of responders v nonresponders was also in favor of amlodipine. Thus amlodipine administered once daily is an effective and safe agent for second-step therapy in mild to moderate essential hypertension.