This double-blind crossover study investigated the efficacy and tolerability of the extended release (ER) form of the calcium antagonist felodipine in 24 patients with mild to moderate primary hypertension using 24-hour ambulatory blood pressure monitoring (ABPM). After a 1-week placebo run-in, patients were randomised to treatment for 1 week with either felodipine ER 5mg or felodipine ER 10mg, both given once daily. At the end of the second week patients were crossed over to the alternative treatment. The 24-hour blood pressure monitoring was performed before randomisation, when on placebo, and at the end of each felodipine ER period. The 5mg dosage reduced supine mean systolic (SBP) and diastolic (DBP) casual blood pressure by 25/11mm Hg and the 10mg dose lowered the pressure by 31/12mm Hg from the placebo period when SBP/DBP was 171/99mm Hg. Both doses of felodipine ER significantly reduced mean 24-hour ABPM supine SBP and DBP compared with placebo. Both day (0600h to 2200h) and night (2200h to 0600h) 24-hour BP profiles showed a significant reduction in SBP and DBP compared with placebo. The trough to peak relationships calculated for both felodipine ER doses from the ABPM were at least 70%. Four patients reported mild vasodilatory adverse events while receiving felodipine ER. In conclusion, analysis of the 24-hour interval indicates that felodipine ER 5 and 10mg once daily is an effective antihypertensive agent that produces a uniform BP reduction that lasts throughout the 24-hour dosage interval.