['To describe the association between body mass index (BMI) and dementia risk in older persons.', 'Prospective population-based study, with 8 years of follow-up.', 'The municipality of Lieto, Finland, 1990/91 and 1998/99.', 'Six hundred five men and women without dementia aged 65 to 92 at baseline (mean age 70.8).', 'Weight and height were measured at baseline and at the 8-year follow-up. Dementia was clinically assessed according to Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, criteria.', 'Eighty-six persons were diagnosed with dementia. Cox regression analyses, adjusted for age, sex, education, cardiovascular diseases, smoking, and alcohol use, indicated that, for each unit increase in BMI score, the risk of dementia decreased 8% (hazard ratio (HR)=0.92, 95% confidence interval (CI)=0.87-0.97). This association remained significant when individuals who developed dementia early during the first 4 years of follow-up were excluded from the analyses (HR=0.93, 95% CI=0.86-0.99). Women with high BMI scores had a lower dementia risk (HR=0.90, 95% CI=0.84-0.96). Men with high BMI scores also tended to have a lower dementia risk, although the association did not reach significance (HR=0.95, 95% CI=0.84-1.07).'] Are overweight and obesity in old age associated with greater dementia risk?