Background: The efficacy and safety of a chronotherapeutic, graded-release diltiazem HCl extended-release (GRD) 120-, 240-, 360- and 540-mg dose administered once-daily at bedtime (10 PM) were evaluated in a 7-week randomized, double-blind comparison to placebo and to GRD 360 mg administered once-daily at 8 AM in 478 patients with moderate-to-severe essential hypertension. Methods: We assessed the change from baseline to end point in trough diastolic blood pressure (DBP) at 6 PM to 10 PM and in mean DBP from 6 AM to 12 noon between GRD 360 mg PM and GRD 360 mg AM, measured by ambulatory BP monitoring (ABPM). Results: Bedtime doses of GRD showed dose-related mean reductions in trough DBP that were significant for GRD doses of 240 mg and higher. Bedtime GRD 360 mg was associated with a significantly greater reduction in mean DBP between 6 AM and 12 noon compared to morning GRD 360 mg with a least squares mean for treatment difference of -3.3 mm Hg (P = .0004). Similar dose-related and significant reductions in systolic BP (SBP) and heart rate (HR) were obtained. Incidence of adverse events (AEs) for all GRD groups (44.5%) was less than that obtained for the placebo group (49.3%). The 540-mg group showed an incidence of AEs (43.5%) similar to that observed for the 240-mg group (42.6%). Conclusions: The GRD dose-dependently significantly reduces BP and HR over the 24-h interval after once-daily bedtime dosing. Further greater reductions were obtained between 6 AM and 12 noon, when circadian BP is highest, compared to morning administration of the same dose. The 540-mg GRD was safe, well tolerated, and offers further therapeutic option for patients with severe hypertension who required additional BP control. © 2003 American Journal of Hypertension, Ltd.