In a meta-analysis of 19 prospective studies with at least 12 months of follow-up the association of smoking with dementia and cognitive decline was assessed. Studies included a total of 26,374 participants followed for dementia for 2-30 years and 17,023 participants followed up for 2-7 years to assess cognitive decline. Mean study age was 74 years. Current smokers at baseline, relative to never smokers, had risks of 1.79 (95% confidence interval (CI): 1.43 to 2.23) for incident Alzheimer's disease, 1.78 (95% CI: 1.28, 2.47) for incident vascular dementia, and 1.27 (95% CI: 1.02, 1.60) for any dementia. Compared with those who never smoked, current smokers at baseline also showed greater yearly declines in Mini-Mental State Examination scores over the follow-up period (effect size (beta) = -0.13, 95% CI: -0.18, -0.08). Compared with former smokers, current smokers at baseline showed an increased risk of Alzheimer's disease (relative risk = 1.70, 95% CI: 1.25, 2.31) and an increased decline in cognitive abilities (effect size (beta) = -0.07, 95% CI: -0.11, -0.03), but the groups were not different regarding risk of vascular dementia or any dementia.