Background identification of modifiable risk factors is crucial in the prevention of dementia, given its limited treatment options. Studies on increased body mass index (BMI) as a risk factor for dementia show conflicting results. Methods we systematically retrieved and reviewed longitudinal population-based studies on increased BMI and dementia using a standard protocol. We searched Medline (1966–2006), Ageline (1978–2006), PsychInfo (1966–2006), CINAHL (1982–2006), and other relevant databases, including the reference lists of theeligible aricles for review. Included studies were subjected to a quality assessment protocol. Results we identified eight studies that met our selection criteria. These studies covered 1,688 cases of dementia from 28,697 participants. After adjustment for age, smoking, comorbidities, and other confounders, four studies presented significantly increased risk of dementia with elevated BMI. Conclusion this systematic review supports the hypothesis that increased BMI is independently associated with increased risk of dementia. Long-term studies to examine the mechanisms underlying the relationship between obesity and dementia are needed.