Vitamin D is a hormone with effects not only on the skeleton, but on most tissues in the body. Lack of vitamin D is associated with cardio-vascular disease (CVD) and type 2 diabetes, and also with risk factors for these diseases like hypertension, dyslipidemia, insulin resistance, and endothelial dysfunction. However, intervention studies with vitamin D have been inconclusive regarding diseases and risk factors. Most of these studies were done in white, Western populations in subjects fairly vitamin D sufficient, and accordingly, no benefits were to be expected. Also, in many studies the doses of vitamin D have been too low, and the studies underpowered. To firmly establish the role of vitamin D regarding CVD risk factors we will in the present study include 600 subjects with vitamin D deficiency (serum 25-hydroxyvitamin D (25(OH)D) < 30 nmol/L) and randomize to high dose vitamin D (3000 IU per day) versus placebo for four months. The subjects will be recruited based on 25(OH)D measurements in the forthcoming 7th survey in the Tromsø study where more than 20 000 subjects are expected to attend. If our hypotheses are correct and the vitamin D supplement has a positive effect, this will be of great importance not only in countries with low sun exposure, but particularly for subjects in developing countries where vitamin D deficiency is highly prevalent.