Rural-urban differences in the association between disability and body mass index among the oldest-old in China.

•Association between BMI and disability differed according to residence.•BMI is a significant indicator of disability in urban seniors (80+).•Both overweight and underweight are red flags of disability among urban oldest old.•Annual physical examination should be recommended to all of the oldest-old.•Drinking is beneficial for the maintenance of ADL among the urban oldest-old.