OBJECTIVE To compare the efficacy and tolerance of felodipine-ER and nifedipine OROS, both once daily, in the treatment of mild-to-moderate uncomplicated arterial hypertension (AH). METHODS This was a multicentric, opened, randomized, paralled trial, that selected 121 patients with uncomplicated, mild to moderate essential AH (diastolic blood pressure (DBP) > or = 95 and < or = 110 mmHg; not under anti-hypertensive medication. All patients received placebo for two weeks. After that period, they would take either 5mg/day of felodipine, or 30mg/day of nifedipine OROS, both once daily, in a randomized fashion. Patients underwent laboratory tests and electrocardiogram (ECG) at the begining and at the end of the study, and heart rate and blood pressure (BP) measurements, nearly 24 hours after the last active drug dose. RESULTS Completed the study 111 patients, 60 in the felodipine group and 51 in the nifedipine group. Compared to baseline, the average of systolic BP and DBP decreased from 162.5 +/- 14.3mmHg and from 102.2 +/- 5.1mmHg to 143.3 +/- 14.6 and 87.9 +/- 7.2mmHg, respectively, at the end of the treatment in the felodipine group; and from 160.5 +/- 16.3mmHg and 102.5 +/- 6.2mmHg to 136.1 +/- 14.2 and 86.7 +/- 7.0mmHg, respectively in nifedipine group (p < 0.0001 for all diferences). Adequate BP response to the treatment (DBP normalization or reduction > 10mmHg from baseline) occured in 47/60 (78.3%) patients in the felodipine group and in 38/51 (74.5%) in the nifedipine group (NS). Side effects, occured in approximately 15% of the cases, and were similar in both groups. These were usually moderate and transient, but were responsible for the withdrawal from the study of two cases in the felodipine group and of three cases in the nifedipine group. CONCLUSIONS Felodipine-ER and nifedipine OROS, are similarly effective and generally well tolerated in patients with mild-to-moderate essential hypertension.