The efficacy and safety of the dihydropyridine calcium antagonists amlodipine and nitrendipine as single-agent therapy of mild to moderate hypertension were compared in an open, parallel-group study. Interim analysis of data from 74 patients (43 male, 31 female) from an expected final total of 96 patients is reported. Amlodipine normalized blood pressure (< or = 90 mm Hg) in 94.7% of patients with a mean final dose of 8.3 mg/day, compared with normalization of blood pressure in 83.3% of patients treated with nitrendipine with a mean final dose of 28.3 mg/day. Only nitrendipine produced a statistically significant increase in heart rate after 2 and 4 weeks of therapy. Nitrendipine-treated patients reported more adverse events (47.2%) than the amlodipine-treated group (26.3%). Two patients from the nitrendipine group discontinued treatment due to treatment-related adverse events and one patient required a dose reduction. In the amlodipine-treated group, all adverse events were mild to moderate and dose reduction was required in one patient. In conclusion, although amlodipine and nitrendipine have comparable antihypertensive efficacy, in this study amlodipine was associated with fewer adverse effects.