The safety and effectiveness of a once daily formulation of diltiazem hydrochloride (diltiazem CD) in the treatment of essential hypertension was assessed in a total of 127 patients with supine diastolic blood pressures (DBP) of 95 to 110 mmHg randomized to diltiazem CD (n = 61) or placebo (n = 66). Patients were titrated to doses of 120, 240, or 360 mg to achieve DBP reduction to less than 90 mmHg. At end study diltiazem CD changed trough supine SBP and DBP by -8.4 +/- 1.7 (p = 0.0009) and -8.6 +/- 1.1 mmHg (p = 0.0075), respectively. Heart rate was not significantly changed (-1.3 +/- 1.1 beats/min, p = 0.4362). The average dose of diltiazem CD was 268 mg with 69% achieving a clinical response. A subset of 47 patients underwent ambulatory blood pressure monitoring to assess the consistency of the effect over the full 24-h dosing interval. Diltiazem CD lowered DBP and SBP throughout the dosing interval. The overall side effect profile was similar to placebo. This study provides evidence of 24-h efficacy of this new, once daily formulation of diltiazem.