The purpose of this study, designed as an open multicenter trial, was to test the antihypertensive efficacy, patient acceptability, and side effects of long-term treatment with slow-release nifedipine in a large population. The drug was studied in 330 outpatients with essential hypertension, WHO stage 1-2, recruited in 20 hospital centers. After washout period was completed, nifedipine (20 mg bid) was given for 1 month (phase 1). Then, the treatment was extended for 4 months (phase 2) with variable doses (range 20-80 mg daily). No other antihypertensive drugs were administered during phase 1. However diuretics, beta blockers, or captopril were added to nifedipine during phase 2 in 11 patients. Seventy patients did not meet criteria for inclusion at washout. During phase 1 and 2, 66 additional patients were excluded due to side effects, the need of other antihypertensive drugs, or non-compliance. Systolic blood pressure significantly lowered (10% or more) in 84% patients in phase 1 and in 76% in phase 2. No responders were 6.1% and 3.6%, respectively. Diastolic blood pressure was normalized in 60% of patients after 5 months of therapy. Effects on blood pressure were equal in young patients and in the elderly, but a minimal rise in heart rate was recorded in younger patients. At least one side effect occurred in 46.6% patients, mainly headache (15.4%), hot flashes (13.3%), ankle edema (12.8%), or palpitation (6.6%). Sixteen patients (8.2%) were obliged to stop nifedipine treatment due to the severity of the side effects. This trial confirms the efficacy of nifedipine in hypertension, both in young and in aged patients.(ABSTRACT TRUNCATED AT 250 WORDS)