Vitamin D receptors have been detected in vascular smooth muscle cells, and 1,25-dihydroxyvitamin D inhibits the renin mRNA expression. Epidemiological studies show an inverse relation between serum 25-hydoxyvitamin D levels and blood pressure, and low serum 25-hydoxyvitamin D levels are reported to be predictors of future development of hypertension. This may indicate an important role for vitamin D in blood pressure regulation. In the present study, 25-hydoxyvitamin D was measured in sera collected in 1994 from 4125 subjects who did not use blood pressure medication, and thereafter measurement was repeated in 2008 for 2385 of these subjects. In sera from 1994 there was a significant decrease in age, body mass index, and systolic blood pressure and a significant increase in physical activity score across increasing 25-hydoxyvitamin D quartiles. After adjusting for sex, age, body mass index, and physical activity, the difference in systolic blood pressure between the lowest and highest serum 25-hydoxyvitamin D quartiles was 3.6 mm Hg. After adjustment for confounders, serum 25-hydoxyvitamin D from 1994 did not predict future hypertension or increase in blood pressure, nor was there any significant association between change in serum 25-hydoxyvitamin D from 1994 to 2008 and change in blood pressure. Our results do not support a causal role for vitamin D in blood pressure regulation, and large randomized clinical trials, preferably including subjects with hypertension and/or low serum 25-hydoxyvitamin D levels, are greatly needed to clarify whether vitamin D supplementation affects the blood pressure.