Abstract Aims: We examined the individual association between body mass index (BMI) and cognitive function among the very elderly. Methods: The present study analyzed data from a survey that was conducted on all residents aged 90 years or more from a district which had 2,311,709 inhabitants in 2005. The subjects were divided into 4 groups according to quartiles of BMI (<16.6, 16.6–18.9, 18.9–21.1