INTRODUCTION: Hypovitaminosis D has been associated with several chronic conditions; yet, its association with cognitive decline and the risk of dementia and Alzheimer's disease (AD) has been inconsistent. METHODS: The study population consisted of 916 participants from the Three-City Bordeaux cohort aged 65+, nondemented at baseline, with assessment of vitamin D status and who were followed for up to 12 years. RESULTS: In multivariate analysis, compared with individuals with 25(OH)D sufficiency (n = 151), participants with 25(OH)D deficiency (n = 218) exhibited a faster cognitive decline. A total of 177 dementia cases (124 AD) occurred: 25(OH)D deficiency was associated with a nearly three-fold increased risk of AD (hazard ratio = 2.85, 95% confidence interval 1.37-5.97). DISCUSSION: This large prospective study of French older adults suggests that maintaining adequate vitamin D status in older age could contribute to slow down cognitive decline and to delay or prevent the onset of dementia, especially of AD etiology.