Objective:To explore the relationship between blood urea nitrogen to creatinine ratio and frailty in the elderly aged ≥65 years in 8 longevity areas in China.Methods:Participants were recruited from the Healthy Aging and Biomarkers Cohort Study. Based on baseline information about blood urea nitrogen and risk for frailty obtained at follow-up of the participants, blood urea nitrogen to creatinine ratio was classified according to quintiles, Cox proportional hazard regression models were used to analyze the association between blood urea nitrogen to creatinine ratio and frailty.Results:A total of 1 562 participants aged (81.0±17.0) years were included, in whom 814 (52.1%) were men, and 258 frailty events occurred during a mean follow-up of (3.73±1.43) years. Cox proportional hazards model showed that after adjusting for relevant confounders, compared with the participants in the lowest quintile group ( Q1), the risk for frailty decreased by 36%, 44%, and 40% in the participants in the third quintile group ( Q3), the fourth quintile group ( Q4) and the highest quintile group ( Q5) respectively [hazard ratio ( HR)=0.64, 95% CI: 0.43-0.94; HR=0.56, 95% CI: 0.38-0.84; HR=0.60, 95% CI: 0.41-0.88]. The risk for frailty decreased by 20% for every unit standard deviation increase in blood urea nitrogen to creatinine ratio ( HR=0.80, 95% CI: 0.70-0.91). Moreover, blood urea nitrogen to creatinine ratio and the risk for frailty showed a nearly linear dose-response relationship. Conclusions:The increase in blood urea nitrogen to creatinine ratio was associated with higher risk for frailty. Maintaining high blood urea nitrogen to creatinine ratio is important for the prevention of frailty in the elderly.