AIMS We aimed at determining the effect of BMI on functional health among older Germans longitudinally. METHODS Data from four waves (2002-2014) of the German Ageing Survey ('Deutscher Alterssurvey'; DEAS), a representative sample of community-dwelling individuals aged 40 years and above, were used. Functional health was quantified by the subscale 'physical functioning' of the 36-Item Short Form Health Survey (SF-36). Fixed effects regressions were used to estimate the predictors of functional health. Linear, quadratic, and cubic terms were included for BMI (self-reported). RESULTS Fixed effects regressions showed significant linear, quadratic, and cubic effects of BMI on functional health in the total sample and in both sexes. Furthermore, regressions revealed that functional health decreased with increasing age in the total sample and in both sexes. In addition, changes in marital and employment status were significantly associated with changes in functional health in men, but not in women. CONCLUSION Our data indicate that the greater the extreme of BMI (either higher or lower), the greater the risk for functional decline. Nutrition programs aimed at preventing changes to extreme BMI might be productive.