For the Cox proportional hazard model analyses, smoking was defined as a categorical variable with the following 6 levels: never smokers, former smokers, and current smokers who smoke less than 0.5 pack per day, 0.5 to 1 pack per day, 1 to 2 packs per day, and more than 2 packs per day. In the Cox models, the never smokers were used as a reference group. The models were first adjusted for the potential sociodemographic confounders age (as a time scale), sex, education (categorized as high school, trade school, college 1-2 years, college 3-4 years, and postgraduate, with grade school as a reference), race (entered as white, Asian, and other, with African American as a reference group), and marital status (classified as never married and divorced/widowed/separated, with married as a reference group). Second, the models were also adjusted for midlife BMI, hyperlipidemia, diabetes, hypertension, heart disease, and stroke (yes/no) during the follow-up. Finally, the analyses were additionally adjusted for midlife alcohol drinking (classified as former, occasionally, 1-2 drinks per day, and >3 drinks per day, with never drinkers as a reference group). To examine whether the association between smoking and dementia, AD, or VaD was modified by sex or race, interaction terms were added to the Cox models.