The blood pressure response of 48 hypertensive persons and 32 normotensive persons to elemental calcium (as the carbonate or citrate salt), 1000 mg/d for 8 weeks, was assessed in a randomized, double-blind, placebo-controlled, crossover trial. Compared with placebo, Ca2+ significantly lowered supine systolic blood pressure by 3.8 mm Hg, standing systolic blood pressure by 5.6 mm Hg (p less than 0.02), and supine diastolic blood pressure by 2.3 mm Hg (p less than 0.05) in hypertensive persons. The response in normotensive persons differed significantly from that in hypertensives (p less than 0.03) as their blood pressure was unchanged. Twenty-one (44%) hypertensive and 6 (19%) normotensive persons achieved a reduction in standing systolic arterial pressure of 10 mm Hg or greater. Reported adverse effects were similar between calcium and placebo phases and did not necessitate withdrawal of any patient from the trial. Treatment with 1000 mg/d of oral Ca2+ for 8 weeks represents a safe, well-tolerated, nonpharmacologic intervention that lowers blood pressure in selected patients with mild to moderate hypertension.