['This investigation sought to examine potential gender differences in the relationship between body mass index (BMI) and functional decline.', 'Cohort study.', 'Rural Pennsylvania.', 'Medicare managed-risk program participants (aged > or =65) in the Geisinger Health Plan. Mean age at study baseline was 71. Final analyzable sample was 2,634 participants.', 'Self-reported weight, weight change, living and eating habits, alcohol and medication use, depression, dentition, and functional status were obtained upon enrollment and again between 3 and 4 years later. Measured height and weight were also recorded at enrollment. Functional decline was defined as any increase in reported limitations in activities of daily living or instrumental activities of daily living over the study period. Logistic regression was used to evaluate the relationship between BMI, as defined by current National Institutes of Health categories, and risk of functional decline while controlling for age, depression, and polypharmacy. The referent category was BMI 18.5 to 24.9.', 'Women had a higher prevalence of reported functional decline than men at the upper range of BMI categories (31.4% vs 14.3% for BMI > or =40). Women (odds ratio (OR) = 2.61, 95% confidence interval (CI) = 1.39-4.95) and men (OR = 3.32, 95% CI = 1.29-8.46) exhibited increased risk for any functional decline at BMI of 35 or greater. Weight loss of 10 pounds and weight gain of 20 pounds were also risk factors for any functional decline.'] Is obesity associated with functional decline in community-dwelling rural older persons?