In this randomized, double-blind, placebo-controlled trial we plan to enrol 200 vitamin D deficient hypertensive patients. Our main objective is to evaluate whether vitamin D3 supplementation with 2,800 IU daily for 8 weeks has an effect on 24-hour systolic ambulatory blood pressure (ABP) compared to placebo. In addition, we also aim to evaluate whether vitamin D3 supplementation alters 24-hour diastolic ABP, pulse wave velocity, N-terminal-pro-brain natriuretic peptide (NT-pro-BNP), corrected QT interval (Bazett formula), renin, aldosterone, 24-hour urinary albumin excretion, HOMA-IR (HOmeostatic Model Assessment: Insulin Resistance), triglycerides and HDL-cholesterol.

Inclusion Criteria:

• 25-hydroxyvitamin D levels below 30 ng/ml (75 nmol/L)
• Arterial hypertension defined according to recent guidelines as an average office blood pressure on at least two occasions of systolic ≥ 140 and/or diastolic ≥ 90 mmHg or a mean 24-hour ABP of systolic ≥ 125 and/or diastolic ≥ 80, or a home blood pressure of systolic ≥ 130 and/or diastolic ≥ 85 or ongoing antihypertensive treatment which was initiated due to arterial hypertension
• Age of ≥ 18 years
• Written informed consent.

Exclusion Criteria:

• Hypercalcemia defined a serum calcium >2.65 mmol/L
• Pregnancy or lactating women
• Drug intake as part of another clinical study
• Acute coronary syndrome or cerebrovascular events in the previous 2 weeks
• Glomerular filtration rate (GFR) according to the MDRD formula < 15 ml/min/1.73m²
• Systolic 24-hour ABP > 160 mm Hg or < 120 mm Hg
• Diastolic 24-hour ABP > 100 mm Hg
• Change of antihypertensive treatment (drugs and/or lifestyle) in the previous 4 weeks or planned changes of antihypertensive treatment during the study
• Any disease with an estimated life expectancy below 1 year
• Any clinically significant acute disease requiring drug treatment
• Chemotherapy or radiation therapy during the study
• Regular intake of more than 880 International Units (IU) vitamin D per day in the last four weeks before study entry or during the study