BACKGROUND: Epidemiological studies suggest that elevated blood urea nitrogen (BUN) and reduced serum albumin could independently predict adverse clinical outcomes in patients with chronic obstructive pulmonary disease (COPD). However, the predictive performance of BUN-albumin ratio (BAR) in critically ill patients with COPD remains to be confirmed. This study aimed to investigate the association between BAR and all-cause mortality in intensive care unit (ICU) patients with COPD. METHODS: This was a retrospective study that included COPD patients with BUN and serum albumin value on the first day of each ICU admission and data were obtained from the eICU Collaborative Research Database. The included COPD patients were divided into three groups stratified by BAR tertiles (T1-T3). Multivariate logistic regression and Cox proportional hazards models were used to examine the association between BAR and all-cause in-hospital and ICU mortality, respectively. Kaplan-Meier curves were plotted to evaluate survival differences among three groups and discrepancies were compared with the log-rank test. RESULTS: A total of 4037 patients were included in the final analysis and the in-hospital and ICU mortality rates were 11.79% and 6.51%, respectively. The multivariate logistic regression analyses showed that continuous BAR was a significant risk predictor of in-hospital mortality (OR: 1.039, 95% CI: 1.026-1.052, CONCLUSION: High level of BAR is associated with the increasing all-cause mortality in critically ill patients with COPD. As an innovative and promising biomarker, BAR might be useful in predicting high risk of death in patients with COPD.