Efficacy and tolerability of antihypertensive monotherapy with the calcium antagonist nitrendipine were investigated in a 6-month open trial in 495 patients with mild to moderate essential hypertension from 101 practicing internists and general practitioners. Previous antihypertensive therapy (57.4%) was stopped for 1 week and therapy then started with nitrendipine, 20 mg once daily. Sixty-one patients discontinued therapy prematurely because of unwanted effects, mostly characteristic with dihydropyridines (headaches, flushing, and ankle edema), and 23 patients because of insufficient efficacy. In 75% of the remaining 411 patients, the goal blood pressure was achieved by nitrendipine monotherapy (10 mg in 17.6%, 20 mg in 73.3%, and 20 mg b.i.d. in 8%) and diastolic blood pressure was between 90 and 95 mm Hg in another 6%. The reduction in blood pressure did not result in changes of heart rate or weight. Nitrendipine was effective in patients of all age groups but patients older than 60 years of age showed a significantly greater fall in systolic pressure than middle-aged or young patients. At the end of the study, 15 patients still reported side effects. Nitrendipine appears to be well suited for first-line therapy of mild to moderate essential hypertension.