Level of adiposity is linked to dementia in epidemiological studies. Overweight and obesity in mid- and late-life may increase risk for dementia, whereas decline in body weight or body mass index (BMI) and underweight in years preceding and at the time of a dementia diagnosis may also relate to dementia. Longitudinal studies with sufficient follow-up are necessary to estimate trajectories that allow better understanding of the relationship between adiposity indices and dementia over the life course. We evaluated the natural history of BMI in relationship to clinical dementia over 37 years in the Prospective Population Study of Women (PPSW) in Sweden. PPSW is a systematic sample of 1462 women born 1908, 1914, 1918, 1922, and 1930 and aged 38-60 years at baseline. Examinations occurred in 1968, 1974, 1980, 1992, 2000, and 2005. Statistical analyses were conducted using mixed effects regression models. Trajectories of BMI over 37 years as a function of age differed between women who did versus did not develop dementia. Women developing dementia evidenced a lesser increase in BMI from age 38 to 70 years. After age 70, the BMI slope decreased similarly (no "accelerated decline") irrespective of dementia status. A lower BMI before and during dementia onset was observed. Women with similar BMI at mid-life exhibited a different pattern of BMI change as they approached late-life that was related to dementia onset. BMI may be a potential marker of dementia-related neuropathologies in the brain. Dementia is related to a common risk factor, BMI, from mid-to late-life.