This study aimed to determine whether the blood urea nitrogen to serum albumin (B/A) ratio is a useful prognostic factor of mortality in patients with aspiration pneumonia. The study included patients with aspiration pneumonia who had been admitted to our hospital via the emergency department (ED) between January 1, 2014 and December 31, 2018. The 28-day mortality after the ED visits was the primary end point of this study. The data of the survivors and non-survivors were compared. A final diagnosis of aspiration pneumonia was made for 443 patients during the study period. Significant differences were observed in age, respiratory rate, albumin levels, total protein levels, blood urea nitrogen levels, C-reactive protein levels, glucose, and Charlson comorbidity index scores between the survivor and non-survivor groups. Moreover, the B/A ratio was significantly higher in the non-survivor group than that in the survivor group. The area under the curve for the B/A ratio was 0.70 [95% confidence interval (CI) 0.65-0.74], 0.71 for the PSI (95% CI 0.67-0.76), 0.64 for CURB-65 (95% CI 0.60-0.69), and 0.65 for albumin (95% CI 0.60-0.70) on the receiver operating characteristic curve for predicting mortality within 28 days of the ED visit. Multivariable logistic regression analysis revealed that the B/A ratio (>7, OR 3.40, 95% CI 1.87-6.21, P < 0.001) was associated with mortality within 28 days of the ED visit. The B/A ratio is a simple and potentially useful prognostic factor of mortality in aspiration pneumonia patients.