BACKGROUND High vitamin D status has been hypothesized to protect against dementia. The aim of the current study was to investigate whether serum 25-hydroxyvitamin D (25(OH)D) level predicts dementia risk. METHODS The study was based on the Mini-Finland Health Survey. The study population consisted of 5010 men and women, aged 40-79 years, and free of dementia at baseline. During a 17-year follow up, 151 incident cases of dementia (International Classification of Diseases, revision 8, code 290) occurred, according to population registers. Serum 25(OH)D concentration was determined from serum samples frozen at -20 °C and stored at baseline. RESULTS Among women, these with higher serum 25(OH)D concentrations showed a reduced risk of dementia. The hazard ratio between the highest and lowest quartiles of serum 25(OH)D was 0.33 (95% confidence interval = 0.15-0.73) in women and 0.74 (0.29-1.88) in men, after adjustment for age, month of blood draw, education, marital status, physical activity, smoking, alcohol consumption, body mass index, blood pressure, plasma fasting glucose, serum triglycerides, and serum total cholesterol. CONCLUSIONS The results are in line with the hypothesis that low vitamin D status may be a risk factor for dementia.