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Background: This study examined the associations of body mass index (BMI) and waist circumference (WC), as well as their short‐ and long‐term changes over time, with incident dementia in older individuals. Methods: Data came from 18,837 community‐dwelling individuals aged 65+ years from Australia and the United States, who were relatively healthy without major cognitive impairment at enrolment. Anthropometric measures were prospectively assessed at baseline, as well as change and variability from baseline to year two (three time‐points). In a subgroup (n = 11,176), self‐reported weight at age 18 and 70+ years was investigated. Dementia cases satisfied DSM‐IV criteria. Cox regression was used to examine the associations between anthropometric measures and incident risk of dementia. Results: Compared to normal weight, an overweight (HR: 0.67, 95%CI: 0.57–0.79, p < 0.001) or obese BMI (HR: 0.73, 95%CI: 0.60–0.89, p = 0.002), or a larger WC (elevated, HR: 0.71, 95%CI: 0.58–0.86, p < 0.001; highly elevated, HR: 0.65, 95%CI: 0.55–0.78, p < 0.001; relative to low) at baseline was associated with lower dementia risk. In contrast, substantial increases in BMI (>5%) over 2 years after baseline were associated with higher dementia risk (HR: 1.49, 95% CI: 1.17–1.91, p = 0.001). Increased dementia risk was also seen with an underweight BMI at baseline and a 2‐year BMI decrease (>5%), but these associations appeared only in the first 4 years of follow‐up. Compared to normal weight at both age 18 and 70+ years, being obese at both times was associated with increased dementia risk (HR: 2.27, 95%CI: 1.22–4.24, p = 0.01), while obesity only at age 70+ years was associated with decreased risk (HR: 0.70, 95%CI: 0.51–0.95, p = 0.02). Conclusions: Our findings suggest that long‐term obesity and weight gain in later life may be risk factors for dementia. Being underweight or having substantial weight loss in old age may be early markers of pre‐clinical dementia. [ABSTRACT FROM AUTHOR]