Pneumonia is a leading cause of mortality. Severity-assessment scores in pneumonia guide treatment crucially, but the ones currently in existence are limited in their use. Community-based studies demonstrated the association between pre-existing low estimated glomerular filtration rate (eGFR) and outcomes in pneumonia. However, whether a single emergency department-eGFR measurement could predict outcomes in pneumonia remains unclear. This retrospective cohort study included 1554 patients hospitalized with pneumonia. The predictor was the first eGFR measurement. Outcomes included mortality, intensive care unit (ICU) admission, durations of hospital and ICU stay, and ventilator use. Receiver operating characteristic curves was used to determine optimal cutoff values to predict mortality. Of 1554 patients, 263 had chronic kidney disease, demonstrated higher C-reactive protein and SMART-COP scores, and had more multilobar pneumonia, acute kidney injury, ICU admission, and mortality. Patients with higher pneumonia severity scores tended to have lower eGFR. For predicting in-hospital mortality, the optimal eGFR cutoff value was 56 mL/min/1.73 m2. eGFR < 56 mL/min/1.73 m2 had an odds ratio of 2.5 (95% confidence interval, 1.6-4.0) for mortality by multivariate logistic regression. In Conclusion, eGFR < 56 mL/min/1.73 m2 is an independent predictor of mortality, indicating that even mild renal impairment affects the outcome of pneumonia adversely.